Djokovic went into the game on an 18-game winning streak and he made it 19 with a 6-3 3-6 6-3 victory over Argentina's Del Potro.
Read how it all unfolded in our game-by-game updates below.
8pmEvening folks! This is arguably the game of the competition so far as two men in tip-top form face off at the spectacular O2 Arena.
8.01pmThe two men are nervously shifting their feet about backstage at the moment as they await their boxing-style entrance in front of an expectant, buzzy crowd.
8.03pmBefore we talk about this match, let's just recap what happened earlier on in the afternoon session. Both Roger Federer and Richard Gasquet had lost their opening matches to Djoko and DelPo respectively and it was Federer who bounced back with a straight-sets win over the Frenchman.
8.05pmDel Potro wins the coin toss in the middle and elects to receive. An odd choice considering he has a big serve but perhaps he feels more confident being served at early on.
8.07pmWhen these two last met in the final of the Shanghai Masters last month it was Djokovic who came through in three sets.
8.10pmBoth men can qualify for Sunday's semi-finals tonight. Any kind of win will take Djoko through but Del Potro needs to win in two straight sets to ensure his place in the last four.
8.12pmSTAT! You have to say that the form books favour Novak tonight. He is on an incredible 18-match winning streak and has won 10 of the 13 matches he's played against Del Potro.
8.14pmOK, here we go... Djoko to serve first.
8.15pmTwo loose backhands from Novak and he's already looking a little shaky at 0-30.
8.18pmStunning 29-shot rally as Djokovic saves a second break point to bring it back to deuce. Del Potro seems to have issues moving off his forehand at the moment as he keeps slipping over.
Djokovic 1-0* Del PotroThe Serb comes through with a big serve to earn advantage and then crisp forehand to clinch the game. Not the best start from the world number two, but he's held.
8.22pmBoth men are feeling it early on and they produce another rally of big-hitting that Djokovic again wins to half the deficit in this game to 30-15.
Djokovic *1-1 Del PotroA simpler hold for DelPo as he tracks down Djokovic's mishit return at 40-15 before the Serb nets his pass.
Djokovic 2-1* Del PotroA love-hold for Djokovic as Del Potro shoots long with a couple of returns before Novak ties up the game with an elegant backhand.
8.28pmLook who's at the tennis tonight, it's Manchester City's in-form Argentinian Sergio Aguero:
8.29pmAguero will obviously be supporting fellow countryman Del Potro but he's in trouble now at 30-40...
8.30pmCLOSE! The world number two almost converts but his in-to-out forehand curls just past the sideline. Back to deuce.
8.31pmBooming serve from Del Potro that Djokovic can only get his frame to and that's a second break point saved by the Argentine.
Djokovic *2-2 Del PotroDel Potro comes through, just, as Djokovic pulls a forehand wide. Both men have now had one easy service game and one testing one.
8.37pmDelPo attacks the Djoko second serve and it pays dividends as his next forehand can only be netted by the Serb. 30-30.
Djokovic 3-2* Del PotroDjokovic wins the next two points though to make it five holds in a row to start the match. Nothing in it so far.
8.42pmDel Potro looks to be in trouble when he offers Djokovic plenty of time to pass him at the net but he guesses cross-court and it's right so he has an easy volley to put away. 30-30.
8.43pmA fourth BREAK POINT of the match for Djokovic as Del Potro hits wide after a heavy-hitting rally. Djokovic is winning those long battles every time at the moment.
8.44pmThe Serb misses out again, this time because of an unforced error off the backhand wing.
8.45pmDjokovic saves a game point with a delightful drop shot before earning BREAK POINT as DelPo mishits a backhand.
Djokovic *4-2 Del PotroBREAK! The Serb converts the break at the fifth attempt as he works Del Potro left, then right before the Argentine is forced to go for a win-or-bust forehand that flashes wide. Advantage Novak.
Djokovic 5-2* Del PotroYou might expect Djokovic to be a little wary after getting that break but he's as cool as a cucumber, getting his first serve in so that it's difficult for Del Potro to assert himself. The Argentine now has to win three games in a row to ensure that he doesn't lose the first set.
8.50pmDjokovic is getting his first serve in an impressive 78% of the time, with Del Potro only at 59%.
Djokovic *5-3 Del PotroDel Potro, serving with new balls, rattles off a quick hold to 15 thanks to a couple of aces. Novak will now serve for the first set.
8.54pmTHREE SET POINTS for Novak as Del Potro can only find the net with a forehand on the run...
Djokovic 6-3 0-0* Del PotroSET! What a way to wrap it up as Djokovic delivers a second-serve ACE down the middle.
8.57pmSo that first set going Djokovic's way means that Del Potro cannot guarantee a place in the semi-finals this evening. He'll now have a showdown match with Federer in the final round of group matches.
9pmWe're back underway at the O2 and Del Potro opens up two game points with his fifth ACE of the match.
Djokovic 6-3 *0-1 Del PotroEncouraging start to the second for Del Potro as he does come through to 15. Can he now shorten the points on Djokovic's serve and earn some more break points? He had some early on in the match but has since looked powerless on the world number two's serve.
Djokovic 6-3 1-1* Del PotroA love-hold for Djokovic as he wraps up the game with a flamboyant drive-volley at the net.
9.05pmDel Potro finally wins one of the lengthy exchanges between the two as Djokovic uncharacteristically hits long with a backhand. 30-15.
Djokovic 6-3 *1-2 Del PotroDjokovic produces an outrageous backhand return winner and 40-15 but can't bring it back to deuce as the holds continue in the second set.
9.08pmDel Potro's serve has got noticeably better in this set. He's upped his first-serve percentage to a whopping 82%.
Djokovic 6-3 2-2* Del PotroNo danger on the Djokovic serve at the moment as he holds to 15 thanks to a wild forehand return from Del Potro. Who is going to crack first in this set?
9.13pmLovely, controlled backhand volley from Del Potro moves him to 40-0. Both men looking so assured on serve right now.
Djokovic 6-3 *2-3 Del PotroDjokovic claws it back to 40-30 but then hits long with a backhand to allow Del Potro to hold.
9.17pmNot a good start from Djokovic as he carelessly pushes a short forehand wide. This is the same game where he broke Del Potro in the first set.
9.19pmTWO BREAK POINTS for Del Potro as Djokovic nets...
Djokovic 6-3 2-4* Del PotroBREAK! HUGE stroke of luck for Del Potro as his backhand clips the tape and drops over onto Djokovic's side of the court. He breaks in the sixth game, just like Djokovic did in the first set.
9.21pmAnother bullish rally from two of the world's top five players and again it's Del Potro who gets the better of this one. He's just added a few more mph to his groundstrokes and it's unsettling Novak.
Djokovic 6-3 *2-5 Del PotroFantastic from DelPo as he holds to love to edge within a game of the second set. Looks like we will soon be into a decider.
Djokovic 6-3 3-5* Del PotroDel Potro seems happy to serve it out as he goes powerfully at a couple of returns but can't get them in. It's a love-hold for Djokovic and now it's a huge game. Can DelPo hold serve and take us into a decider?
9.29pmHuge point coming up now at 30-30. Massive...
9.30pmCasual from Djokovic as his off-forehand drifts wide. SET POINT DEL POTRO...
Djokovic 6-3 3-6 *0-0 Del PotroSET! He gets it, banging down a wide serve that Djokovic can only block back long.
Djokovic 6-3 3-6 1-0* Del PotroDjokovic begins the third with a hold to 30. He raced to a 40-0 lead but was then pegged back.
9.39pmGame two of the third set starts with two gorgeous winners from either side of the court. Del Potro's forehand sent Djokovic the wrong way before the Serb responded with a clean backhand down the line.
Djokovic 6-3 3-6 *1-1 Del PotroWe're all level at the start of the third as Djokovic fails to return another big serve from DelPo.
9.42pmDodgy start to his service game from Djokovic as he wildly slaps a drive-volley a long way out before double-faulting. 0-30 and a glimpse of a break for Del Potro.
9.44pmTWO BREAK POINTS now for the Argentine as Djokovic again gets it wrong, this time with a regulation forehand.
9.45pmWe're back at deuce as first Djokovic booms down an ACE before bravely approaching the net, with Del Potro failing to curl a forehand down the line.
Djokovic 6-3 3-6 2-1* Del PotroThat could be a huge hold for the world number two as he rattles off four points in a row to prevent going a break down in the decider. Can Del Potro now recover from that setback and put the pressure back on the Serb?
Djokovic 6-3 3-6 *2-2 Del PotroHe does just that with a confident hold to 15, sealing the game with a mighty serve up the middle that Djokovic can't get back in play.
9.53pmDel Potro guesses right on Djokovic's approach shot and he should at least test the Serb but can only mishit well wide. 30-15.
Djokovic 6-3 3-6 3-2* Del PotroWe remain on serve in the final set as Djokovic calmly puts away the easiest of forehand volleys. Now we come to the sixth game, where in both of the first two sets the deciding break was made. Can Del Potro hold to stop the rot?
9.57pmUncomfortable start to the game from Del Potro as he first shoots long with a backhand before dragging a forehand into the net. 0-30 and worrying times for the Argentine.
9.57pmTHREE BREAK POINTS FOR DJOKOVIC...
Djokovic 6-3 3-6 *4-2 Del PotroBREAK! He takes the first as Del Potro again fires long. The sixth game of the set proves to be the undoing for the server once more.
Djokovic 6-3 3-6 5-2* Del PotroDel Potro offers little fight in that game as Djokovic moves to within one of the match and a place in the last four of the tournament.
10.03pmThe crowd look to get behind Del Potro as he hits a forehand winner to open the game. Has he got any fight left?
Djokovic 6-3 3-6 *5-3 Del PotroThe number four seed forces his opponent to serve it out by holding to 30.
10.07pmNo problems for the Serb on the opening two points as he moves to within two of the match.
10.08pmSTUNNING drop-volley from Djokovic and he has THREE MATCH POINTS...
Djokovic 6-3 3-6 6-3 Del PotroGAME, SET AND MATCH DJOKOVIC!
10.09pmClassy backhand up the line wraps it up for the world number two. He extends his winning streak to 19 games. For Del Potro, he'll now have a whoever-wins-goes-through final match against Roger Federer.
10.12pmAnyway, that's all we've got time for here on SM. Make sure you come back tomorrow for Ferrer vs. Wawrinka and Nadal vs. Berdych. See you then.