With the world number one taking on the six-time champion in early November, fireworks were expected.
You can revisit the explosive action below with Sports Mole's blow-by-blow text commentary.
1.42pmHello and thanks for joining Sports Mole for the first of two mouth-watering singles semi-finals at the O2 Arena.
1.43pmLater today, world number two Novak Djokovic will take on Stanislas Wawrinka in this wonderful theatre, but, right now, we are focusing on the 32nd installment of Rafael Nadal versus Roger Federer.
1.44pmThis match will begin shortly after the conclusion of this afternoon’s doubles semi final, in which David Marrero and Fernando Verdasco are currently winning 7-6 6-5 against Ivan Dodig and Marcelo Melo.
1.46pmBack to the main event though... Nadal has been victorious in 21 of the 31 previous meetings with fellow great Federer, including winning seven of the last nine encounters.
1.47pmHowever, both of Federer’s wins over Nadal during that period have come here at the O2 Arena. The Swiss just loves this event and has reached the final in North Greenwich in each of the last three seasons.
1.48pmNadal is likely to be the fresher of the two players this afternoon. His last match – a 6-4 1-6 6-3 win over Tomas Berdych – was completed on Friday night. In contrast, less than 24 hours have passed since Federer booked his place in the last four with a 4-6 7-6 7-5 success over Juan Martin del Potro.
1.50pmPREDICTION: Nadal breezed through his group opener against David Ferrer, but he was thoroughly tested in his other two round robin matches, against Wawrinka and Berdych. I expect the Spaniard to be tested again, but I think he will progress. I’m going to predict Nadal to win in three. The majority of the High Street bookmakers have made Nadal a 1/3 favourite – that sounds about right.
1.52pmGAME SET AND MATCH: Marrero and Verdasco are through to the doubles final after beating Dodig and Melo 7-6 7-5. The Spanish duo will face either the Bryan brothers or Alexander Peya and Bruno Soares in Monday's final.
1.55pmThe Spaniards among the ever-growing crowd in the Arena will be delighted by that doubles result. However, they are here to see Rafa, who will be on court in just minutes.
2pmFederer, who has beaten Nadal in all four of their previous indoor meetings, has just been speaking to Sky Sports in the tunnel. He said: "I've got a game plan in mind and hopefully I can execute it today."
2.03pmNadal: "I have not found the key yet to play against him indoors. I know I will have to play my best tennis to win today."
2.06pmBoth men enter the Arena, making their way past the smoke machines. They are greeted by a wall of noise, including lots of cheering and whooping.
2.09pmThe players are introduced individually over the sound system and, by my reckoning, Federer gets the slightly bigger cheer.
2.11pmThe toss goes Federer's way - he will serve first.
Nadal *0-1 FedererFederer finished yesterday's match with an ace. He starts this encounter in the same fashion, before moving 30-0 ahead with a crisp serve and volley. The Swiss then mishits a forehand to give Rafa a sniff, but the Spaniard's faint hope of a break is quickly extinguished as Federer holds to 15.
Nadal 1-1* FedererNadal's first serve is on target early on and Federer manages to return just one of Spaniard's four spinning point-starters. The world number one's first ace of the day secures him a love service hold.
Nadal *1-2 FedererFederer races into a 40-0 lead before taking his foot of the gas, allowing Nadal to fight back to deuce. However, the Swiss regains his composure, hammering in an ace and then wrapping up the game with a slick forehand winner into the corner.
Nadal 2-2* FedererMore imperious serving from Nadal, who makes three of four first serves en route to another hold to love.
Nadal *2-3 FedererRight back at you, Rafa! Federer holds to love. The Swiss has been able to keep his service points short so far. He will be hoping that he can continue to do so, as Nadal is the master of long exchanges.
Nadal 3-3* FedererAfter eight successive service points go the way of Rafa, he finally loses one, as Federer takes the opening point of game six when Nadal shanks into the net. The Spaniard responds to move 30-15 ahead, but Fed then gets a good read on his opponent's serve, enabling him to crack a stunning backhand return winner. Nadal then clips the top of the net, sending a forehand long. That brings about the match's first BREAK POINT, which is saved by Rafa. Two more break points then come and go before Rafa secures a gritty hold to 40. VAMOS!
Nadal *3-4 FedererIt is often claimed that a player is most vulnerable in the game directly after they have failed to convert multiple break points. Well, Federer is made of stronger stuff than most players. He channels his frustration perfectly to hold to love. The Swiss has won 16 of 20 service points so far.
Nadal 4-4* FedererNormal service resumed for Rafa, who holds to love to level the match at four games each. Federer has failed to win a single point in three of his four receiving games so far.
Nadal *5-4 FedererNADAL BREAKS! I mentioned earlier that Federer will want to keep his service points short today. He is unable to do so in game nine and Rafa grinds him down with a barrage of topspin forehands into the six-time champion's backhand, which simply cannot cope. Nadal breaks to 15.
Nadal 5-5* FedererFEDERER BREAKS BACK! Federer might want to keep the points short today, but he proves that he is still capable of mixing it with the best as he wins a 30-shot rally to move 15-40 ahead. He then converts his first of two break points when Nadal scoops a forehand long.
Nadal *6-5 FedererNADAL BREAKS! Federer's joy is short-lived. The pendulum swings back in Rafa's favour as he breaks to 15 again. He will now serve for the opening set once more.
Nadal 7-5 0-0* FedererFIRST SET NADAL! Federer is given absolutely no chance on this occasion, as Rafa finds his best form when it really matters, holding to love to clinch the opening set after 44 minutes.
3pmNadal won 17 of the last 23 points in that first set. One feels that the Spaniard has the potential to raise his level further, though. On the other hand, Federer appears to be performing close to his best. If the Swiss dips, this could be over pretty quickly. Let's hope not.
Nadal 7-5 *0-1 FedererNo dipping from Fed in the opening game of set two, as he holds to 15.
Nadal 7-5 1-1* FedererRafa struggles to get good length on his shots, giving Federer opportunities to punish. At 30-30, Fed cracks a glorious crosscourt backhand to set up a simple forehand put-away, which he unforgivably slaps into the net to let Nadal off the hook. The Spaniard eventually holds to 40.
Nadal 7-5 *1-2 FedererFederer's forehand let him down in the previous game. However, this time it gets him out of trouble. At 30-30, he slams a winning crosscourt drive right into the corner. He then finds a timely first serve, which Rafa returns long.
Nadal 7-5 2-2* FedererNadal is famous for taking his sweet time. However, he appears to be in somewhat of a rush right now. He holds to love in a flash, before electing to change his racquet in preparation for Federer serving with new balls.
Nadal 7-5 *3-2 FedererNADAL BREAKS! Federer plays one of the worst shots of his career, as he plunders a simple forehand low into the net to give Rafa a break to 30. The crowd goes strangely quiet. They sense that this match could soon be over. And, what's more, they know that this could be the 17-time Grand Slam champion's last appearance at this fine tournament.
Nadal 7-5 4-2* FedererRafa puts daylight between himself and his opponent with a composed hold to 15.
Nadal 7-5 *4-3 FedererA love service hold for Federer rouses his fans once more. The Swiss produced his sixth ace of the match during game 19. He has hit 27 aces this week.
Nadal 7-5 5-3* FedererRafa thinks that he has hit his third ace of the tournament to move 40-30 ahead, but Hawk-Eye proves him and the line judge wrong. It proves to be irrelevant though, as Federer makes two unforced errors to hand the Spaniard another hold of serve.
Nadal 7-5 6-3 FedererGAME SET AND MATCH NADAL! An anticlimactic end to a very good match, as Federer comes up with two errors to hand Nadal a match-ending break to 30.
3.38pmNadal has now beaten Federer indoors for the first time in his career.
3.41pmThe Spaniard won 57% of points today (65/115). He converted all four of his break points and had an impressive first serve percentage of 77%.
3.43pmFederer's main issue was his second serve, which he had to use in 34% of service points. The Swiss won just eight of 20 points behind his second serve.
3.45pmNadal is through to his second final here in North Greenwich. He has never won this tournament, but he will have the chance to correct that tomorrow when he will face either Novak Djokovic or Stanislas Wawrinka, whose semi-final will take place tonight (not before 8pm).
3.47pmSports Mole will also be providing live text coverage of tonight's clash between Djokovic and Wawrinka, but, for now, I must say goodbye. I will give the final word to Rafa, who has been speaking to Sky Sports.
3.49pmRafa on tomorrow's final: "I am very happy for everything. It's the perfect way to end the season, after one of the best seasons of my career."