Afternoon folks! Should be a good battle this one as second seed Azarenka, who's always there or thereabouts in Slams, faces a returning-to-form Ivanovic.
As mentioned above, this is the only fourth-round match still to be played because of regular rain interruptions throughout the tournament.
The other seven names in the quarter-finals are Serena Williams, Carla Suarez Navarro, Ekaterina Makarova, Li Na, Roberta Vinci, Flavia Pennetta and Daniela Hantuchova.
If Ivanovic upsets Azarenka here then the match against Hantuchova would arguably be the most good-looking tennis match you could dream up!
Vika, as she is sometimes known, is, however, heavy favourite with the bookies and should have enough power and consistency from the baseline to see off 13th seed Ivanovic today.
Both of these women are Grand Slam winners, with Ivanovic victorious at the French Open in 2008 and Azarenka a two-time winner of the Aussie Open in the last two years.
Azarenka is hoping to go one better than last year at Flushing Meadows where she came within two points of winning the title when serving at 5-4 in the deciding set against Serena Williams.
Ivanovic, meanwhile, will equal her best run in New York with a win today.
The two women are coming to the end of their warm-ups now so we will be underway momentarily. Hearing that Ivanovic won the toss and chose to serve. That's quite rare in the for the women as returning is often preferred to serving in the modern game.
"First set, Miss Ana Ivanovic
to serve. Ready? Play," says chair umpire Eva Asderaki from Greece.
We're at 30-30 in the opening game as Ivanovic follows up a first serve with a wrong-footing forehand back behind Aza.
The Serb ends the game with a winners off both wings. Impressive stuff and a confidence-boosting hold to start for the underdog.
It's BREAK POINT Ivanovic as Azarenka's down-the-line backhand falls just out. Both women absolutely hammering the ball at the moment.
BREAK! And break she does as Vika misfires with a forehand. One thing Ivanovic is doing very well at the moment is looping the ball with less pace when she's on the defensive, making it harder for Azarenka to move her out of position quickly.
Ivanovic races to 40-0 thanks to a fortunate net cord, first ACE and then clean backhand winner. She's really playing well, the Serb.
The 13th seed consolidates the break by holding to 15 as Azarenka's simply drive-volley balloons up off the tape and drops long. Everything going Ivanovic's way at the moment.
Ivanovic smacks a return cross-court for her eighth winner to make it 30-15. Amazingly, Vika hasn't got a single one yet.
"Are you freakin' kidding me?!" says Azarenka as umpire Asderaki deems that she should lose the point after Ivanovic successfully challenges a ball that she put on the baseline. The Belarusian isn't in the best of moods at the moment but she needs to keep her mouth shut as it was the right call from the Greek official.
Despite the outburst, Azarenka comes through that game and squeals "COME ON!" to let Ivanovic know that she hasn't given up on this set yet.
The longest rally of the match comes to an end when Ivanovic, who had been behind for almost the whole rally, slaps a forehand winner cross-court with the 15th stroke. That could be huge as she was 0-30 down at the time.
That's four points in a row for Ivanovic as she comes form 0-30 behind to hold serve and move within a couple of games of the first set on Arthur Ashe.
Ivanovic looks dangerous at 15-30 in the next but then blazes a backhand return wide of the tramlines before putting a forehand pass into the net. Game point Vika...
She can't take it as Ivanovic again uses the high top spin to unsettle the number two seed, who has plenty of time for her backhand but hits it miles long.
Wicked return from Ana and she has DOUBLE-BREAK POINT...
She can't take it and now it's game point Vika as the Belarusian puts away a volley for her first winner of the day.
And that's followed by her first ACE! Absolutely vital, that hold, for the second seed.
No danger from Azarenka of breaking back at the moment as Ivanovic breezes through that game to 15 to edge within a game of the opening set. We've played half an hour of tennis on Ashe.
Whooosh! Ivanovic starts the eighth game with a rocket return to move 0-15 ahead. Surely the 13th seed can't keep this up?
Or not! The Serb curls yet another winner cross-court after forcing Azarenka to the net with a low slice.
Azarenka wins the next four points to force Ivanovic to serve it out. At the moment Ivanovic seems to be guessing right when Azarenka has an easy shot every time!
Uh oh! Is Ivanovic about to unravel? She begins the game with loose backhand before double-faulting! 0-30...
THREE BREAK-BACK POINTS AZARENKA...
BREAK BACK! Ivanovic saves the first two but Azarenka grabs the third by coming to net and putting away an easy overhead. Can the Belarusian now keep her cool and serve to stay in the set for a second time?
Gorgeous squash-style slice from Ivanovic on the back foot sees her move 15-30 ahead and within two points of the set.
TWO SET POINTS IVANOVIC! What a forehand!
BREAK AND SET! Whoa! Ivanovic crashes a forehand just over the net and it's simply too hot for Azarenka to handle even though it's not wide of her. Fantastic set from the underdog.
It's a fully deserved first set for Ivanovic, who hit 16 winners to Azarenka's three!
Perhaps this upturn in form is because Ivanovic recently joined Twitter? This was the first tweet that she sent a couple of weeks ago:
BREAK! Not a great start to the second set for Ana as she is edged out after being taken to deuce by Azarenka.
BREAK BACK! Neither woman looks particularly confident on serve at the moment as Azarenka double-faults for a third and then FOURTH time to gift Ivanovic the break back.
Yes, this Arthur Ashe court is susceptible to swirling winds and it will affect the ball toss, but serving should be a strength, not a weakness.
Wow, check out this stat! Forehand winners: Ivanovic 15-0 Azarenka
Ivanovic moves 40-15 ahead on serve thanks to three straight points going back behind her opponent. I mentioned earlier that Ivanovic seemed to be guessing right almost all the time and it's completely the opposite way round for Vika, who can't work out where the Serb is going.
BREAK! Obviously I type the below and she goes and has a poor finish to the game! It's three straight breaks to start the second as Ana double-faults at advantage Azarenka. Ashe is filling up more and more now as people hear of the neck-and-neck battle going on right now.
It's looking like yet another break as Ivanovic races 0-40 ahead thanks to a pair of winners before Vika shoots way long with a backhand.
BREAK BACK! Azarenka saves the first with a big serve but surrenders the second by netting a backhand approach.
No-one looks even close to holding serve at the moment as Ivanovic slumps to 0-40. Three break points Azarenka...
BREAK! Oh for goodness sake! That's now seven straight breaks of serve if you go back to the end of the first set! Ivanovic pulls it back to 30-40 but then double-faults AGAIN on the big point after Azarenka successfully challenged the first serve, which Hawk-Eye showed to be out by a millimetre.
A hold! A hold of serve! It's Azarenka who opens up daylight by holding to 15 in a game that included a delicious drop-shot. Looks like we'll be heading into a deciding third set!
Ivanovic ensures that Azarenka doesn't race clear in the set by holding herself for the first time in five service games. Winners: Ivanovic 27-8 Azarenka, Unforced Errors: Ivanovic 23-17 Azarenka.
Azarenka holds to 30 and is now within a game of taking us into a third set. All those Ivanovic shots that found the lines and corners in the first just aren't coming off now.
DOUBLE BREAK AND SET! The second set is over in a flash as Azarenka breaks to love.
What must be going through Ivanovic's mind? She looked so good to earn her biggest win in recent memory after that first set but now looks in danger of feebly crumbling in this decider.
It'll be Vika to serve first in the decider. She moves 30-0 ahead but Ivanovic swats her 29th winner to get her first point of the set and end the run of points against her.
The Belarusian holds. Massive game coming up for Ivanovic. She needs to hold.
BREAK! Oh dear, this one could get away from Ivanovic very, very quickly if she carries on like this. It's a love-break for Vika as the Serb nets a short forehand.
Signs of life from Ivanovic as she earns BREAK-BACK POINT...
She gets a second serve but returns tamely to allow Azarenka to take charge of the point. Back to deuce.
Azarenka has a chance to move three games clear but then double-faults once, then twice to present Ivanovic with another break-back point...
BREAK BACK! This time she takes it with a magnificent topspin backhand lob! What a shot! The crowd are right on her side now!
Ivanovic really needs to back up that break and we now have a massive point at 30-30 as Azarenka carelessly nets.
It's Ivanovic who takes it by pulling off a down-the-line backhand. Exuberant celebrations follow. Think she may be trying to get inside Vika's head.
It doesn't get to the Belarusian though, who now has break point...
Azarenka hits wide off the backhand wing and we're back to deuce. You feel this a huge game in the context of the match. If Ivanovic wins, it's still very much in the balance but if Aza takes it then you have to favour her.
BREAK! Azarenka pulls off a lovely drop-shot to clinch the break. Will that knock the fight out of Ana?
Doesn't look like it! Ivanovic begins her quest to break back with a perfectly-timed return winner.
BREAK BACK! It doesn't take long at all for the Serb to get back on serve as Azarenka double-faults. It's quite hard to watch two professionals struggling so much to hold their own serve.
Azarenka successfully challenges a deep groundstroke that was called out to keep it at 30-15. She can't pull it back to 30-30 though as Ivanovic bangs in a first serve that she can't put back in play. Two chances for Ana to hold...
She nets on the first but takes the second as Vika curls a forehand into the tramlines. So hard to pick a winner now.
Azarenka holds after a bludgeoning rally at 40-30 ends with Ivanovic dumping a forehand into the net.
Not a good start to the game for Ivanovic as she double-faults for an eighth time.
THREE BREAK POINTS for Azarenka...
Ivanovic saves the first two thanks to two crucial first serves. One left...
BREAK! She hits out with a backhand on the third though and Azarenka will now serve for the match.
Can Ivanovic find some resolve now to stay in this match? Here she is celebrating a winner earlier in the match:© Getty Images
Massive point coming up now at 30-30. Just two more points will do it for Vika.
Great stuff from Azarenka as she stays calm as Ivanovic comes to the net but is beaten by an assured down-the-line pass.
Ivanovic saves the match point by coming through a backhand-to-backhand battle.
And now BREAK-BACK POINT for Ana...
Fair play to Azarenka, she's just played an incredibly accurate drop shot just over the net to save it.
Ivanovic earns another break point with a fantastic forehand winner on the run but doesn't give herself a chance by mishitting the return at advantage. Deuce again.
And now MATCH POINT number two for Vika as Ana nets a return.
She nets! We go on!
BREAK BACK! Ivanovic hangs in there! She saves two match points and must now hold serve to make sure it wasn't all in vain.
Good start from the 13th seed as she plants a down-the-line forehand into the corner. 15-0.
Ana moves 30-0 clear but we're now back to 30-30 as the Serb's volley just drops short. HUGE point...
MATCH POINT number three for Azarenka as Ivanovic drags a backhand wide...
SAVED! Brilliant tennis from Ivanovic as she kicks a serve out wide before going back behind the Belarusian with an in-to-out forehand.
GAME, SET AND MATCH AZARENKA!
It's not who the crowd wanted to win, but Azarenka survives a tough test to reach the quarter-finals at Flushing Meadows.© Getty Images
Next up for the second seed is a match against lanky Slovak Daniela Hantuchova. That match will be played tomorrow and Azarenka may be a little tired after this tough workout.
If you want a short summary of the match then read our report by clicking here
Anyway, thanks for joining Sports Mole
for coverage of this gripping, toe-to-toe match. Keep up with all the scores, results and reaction from Flushing Meadows with us right here