Despite having only beaten Federer on three previous occasions, Argentine Del Potro managed to edge a three-set encounter at the O2 Arena.
Below is our minute-by-minute updates of how the match unfolded.
1.25pmAfternoon all. We've got a really interesting match to get the final group matches underway as defending champion Roger Federer takes on former US Open winner Juan Martin del Potro.
1.29pmIt will be interesting to see how Federer approaches this match. He's already qualified for the semi-finals, but it is yet to be determined if he will top the group or finish second. Victory over JMDP will seal first place, while a defeat in straight sets will see him advance as a runner-up.
1.31pmOf course he may also be taking into consideration who he wants to face for a place in the final. Topping the group will mean a meeting with Andy Murray. Whereas, second spot will see him come up against world number one Novak Djokovic. I bet he wishes that there was a third option!
1.34pmFor Del Potro, it's not quite so simple. A victory in straights sets will see him top the group, but a defeat will leave him hoping that Tipsarevic defeats Ferrer later. A three-set win will also leave him sweating on tonight's match. Best win it easy then!
1.35pmHowever, wins over Federer have not been easy to come by for Del Potro. From 16 meetings, the Argentine has only emerged victorious on three occasions.
1.39pmHaving said that, all three of those wins have arrived when it has really mattered. His first came in the final of the US Open back in 2009 thanks to a 3-6, 7-6(5), 4-6, 7-6(4), 6-2 success - that still ranks as one of my favourite matches in recent times incidentally! In the same year, he also overcome Fed at this event.
1.43pmHis latest win is likely to still be fresh in the memory of both players. The Swiss Open is a tournament that Federer almost always wins! Del Potro had other ideas this year though and managed to defeat the hometown favourite.
1.49pmThere has also been another memorable encounter between these two this year. Despite their Olympic semi-final being just the best of three sets, they somehow made the match last for almost five hours! Federer eventually won through 19-17 in the final set. Not sure my typing hands could handle something similar today!
1.52pmWe've still got a bit of time before these two come onto the court. We're waiting for a doubles match involving the Bryan brothers to end. It's currently in a champions tie-break so shouldn't be too long.
1.57pmAnd the doubles match is done. The Bryan brothers have lost to Stepanek and Paes. The Bryans must now wait for the outcome of tonight's double matches to determine whether or not they go through to the semi-finals. That will be a shock if they miss out.
1.59pmWhile we wait for the players to emerge, take a look at this video clip from the US Open final. Look how hard they strike the ball down the forehand wing.
2.02pmFrench number two Richard Gasquet, who recently faced Del Potro, believes that the Argentine can defeat Federer today: "He can win today. Roger is the favourite but you never know. He is a big server and has a lot of power," Gasquet just told Sky Sports News.
2.05pmDel Potro tells Sky Sports News that he is hoping for a repeat of the Swiss Open: "I have to repeat that match. I played focused and aggressive there. I will need to be at 100%. I've got confidence in myself."
2.06pmMeanwhile, Fed adds: "The last few matches have been extremely difficult with Juan Martin. I'd rather play Andy Murray [in the semi-finals] because that means I've won the group and beaten Juan Martin."
2.07pmHere come the players. You can guarantee that Federer will get a terrific reception. He always does!
2.11pmWell, well. Del Potro obviously wants to claim an early break. He won the toss but has chosen to receive the Federer serve first. It worked for him against Tipsarevic last time out. Federer will be a totally different proposition though!
2.16pmRight, here we go. Federer will get this match off and running.
Federer 1-0* Del PotroJMDP gets himself up to 30, but it was a relatively straightforward hold for Federer. Have to say that Del Potro is standing very deep when returning the serve. It allows Federer to take control of the rally early on.
Federer *1-1 Del PotroPositive response from Del Potro. Both players are striking the ball better already than they have all week at the O2.
Federer 2-1* Del PotroThe first point in that game went the way of Del Potro, but after that it was all bout Federer. He eased his way through and now leads 2-1.
Federer *2-2 Del PotroBrute power gets Del Potro through that game. He may not be the prettiest on the eye, but it is certainly working at the moment. Two apiece.
Federer 3-2* Del PotroI reckon Federer must have dinner reservations in a flash London restaurant! He's not hanging about with a love game there. What's worse for Del Potro, he wasn't really given a sniff. Federer has found his serving range early.
Federer *3-3 Del PotroJust moments after Federer serves out to love for the first time, Del Potro follows suit. The Argentine has only failed to land his first serve in on three occasions so far.
Federer 4-3* Del PotroNeither of these two are messing about at the moment are they?! It's another speedy hold by Federer. We've gone through seven games already!
2.42pmA composed smash brings up two break points for Federer...
2.43pmACE! That's a great way to save one. Still one more remains for Federer.
2.44pmAnd then Del Potro forces Federer to hit long with a powerful forehand. Great character from 'Del Boy'. Deuce.
2.44pmBut Del Potro hits wide and Federer has another break point...
2.45pmOnce again Federer goes long though. His third break point chance evaporates. Deuce.
Federer *4-4 Del PotroThat's some good tennis from Del Potro to dig himself out of a hole. He attacked Federer and forced some mistakes.
2.50pmMore forceful tennis from Del Potro brings up deuce on the Federer serve...
2.53pmACE! He's made to work hard but Federer manages to hold with an inside ace. This one has been all about power so far! Del Potro will now serve to stay in the set.
Federer *5-5 Del PotroA very important hold for Del Potro, with a game that included an ace. This one has tie-break written all over it. In truth, it has for a while.
Federer 6-5* Del PotroThat hold looked easy for Federer - although I'm sure it wasn't! He hit his third ace and is now one behind Del Potro's total for the match. Talking of Del Potro, he will again serve to stay in the set. Pressure.
Federer 6-6 Del PotroWell Del Potro certainly didn't show any signs of pressure there. He served out to love and now we have a breaker...
TIE-BREAK:Federer 1-0 Del Potro*
TIE-BREAK:Federer 1-1 Del Potro*
TIE-BREAK:*Federer 1-2 Del Potro
TIE-BREAK:*Federer 1-3 Del Potro
TIE-BREAK:Federer 1-4 Del Potro*
TIE-BREAK:Federer 1-5 Del Potro*
TIE-BREAK:*Federer 1-6 Del Potro - Five set points Del Potro...
TIE-BREAK:*Federer 2-6 Del Potro - Four set points Del Potro...
TIE-BREAK:Federer 3-6 Del Potro* - Three set points Del Potro...
Federer (3)6-7 0-0* Del PotroSET! Well that's a slack breaker from Federer - not too dissimilar from the one that he produced in Basel against Del Potro. Having said that, there's been very little between these two. Don't be surprised to see this go the distance.
3.14pmThree break points for Federer at the start of this second set...
Federer (3)6-7 *1-0 Del PotroBREAK! That is the perfect response from Federer to losing the first set. He breaks Del Potro to love, which gets a huge cheer from the crowd.
Federer (3)6-7 2-0* Del PotroAnd that is followed by a love hold by Federer. Del Potro is yet to win a point in this set.
Federer (3)6-7 *2-1 Del PotroThat's better from Del Potro. He seems to have rediscovered the forehand stroke that had deserted him at the start of this set. He's now on the board, although Federer has the break.
Federer (3)6-7 3-1* Del PotroComfortable stuff for Federer, who is now two games in front. There's pressure on Del Potro to stay in touch with this service game.
Federer (3)6-7 *3-2 Del PotroWell if there was any pressure, he certainly didn't show it! He's started to attack Federer's body and it has paid off. He still has to somehow break Federer though.
Federer (3)6-7 4-2* Del PotroAnother love hold for Federer. He's cruising towards this set at the moment because Del Potro hasn't had a sniff of a break.
Federer (3)6-7 *4-3 Del PotroFederer gets himself up to 30 but tries to be a little too forceful and shoots long, thus giving Del Potro the hold. Fed still has that important break in the bag though.
Federer (3)6-7 5-3* Del PotroAnother routine hold for Fed leaves him just one game away from taking this set and forcing a decider. Del Potro now serving to stay in it.
Federer (3)6-7 *5-4 Del PotroACE! Another ace seals the game for Del Potro. Now comes the biggest game of the match so far. If Fed holds, we are heading for a decider.
3.44pmFederer has a set point...
Federer (3)6-7 6-4 0-0* Del PotroACE! SET! The perfect way to sign off from that set by Federer. He was on top form throughout that one and we will now have a decider. That also means that he will top this group regardless of the result. Andy Murray awaits in tomorrow's semi.
Federer (3)6-7 6-4 *0-1 Del PotroPositive stuff at the start of the third set by Del Potro. He moved Federer around the court a lot and reaped the benefits. He's 1-0 up.
3.53pmWell, well. A sweeping backhand down the line brings up break point for Del Potro...
Federer (3)6-7 6-4 0-2* Del PotroBREAK! He's got it! Federer was up 30-0 in that game but Del Potro has battled back to claim a crucial break. What a start for him to the decider!
Federer (3)6-7 6-4 *0-3 Del PotroFantastic tennis from Del Potro, who serves out to love. It looks a long way back for Federer now. However, if anyone can get back into this, it's the world number two!
Federer (3)6-7 6-4 1-3* Del PotroR-Fed gets himself involved in this set with a comfortable hold. All that matters though is the Del Potro serve. If he holds three more times, he's won the match.
Federer (3)6-7 6-4 *1-4 Del PotroNet play is not something that Del Potro has always been associated with but he showed his ability close up during that game. At one point he shaped to go across the court, before sending the ball down the line and beyond a wrong-footed Federer. He still hasn't dropped a point on his serve in this set. What's more, he's two games away from victory.
Federer (3)6-7 6-4 2-4* Del PotroTwo aces from Federer bring up his second hold. It's on the Del Potro serve where he needs to make inroads though. He's running out of time.
Federer (3)6-7 6-4 *2-5 Del PotroFederer managed to get his first point on the Del Potro serve but that is all he got. He's cutting a frustrated figure at the moment. Now serving to stay in the match.
Federer (3)6-7 6-4 3-5* Del PotroACE! Another ace from Federer brings up the hold and puts pressure on Del Potro, who is now serving to win this match.
4.16pmACE! Stunning serve from Del Potro brings up 30-0. Almost there.
4.17pmTwo match points for Del Potro at 40-15...
4.18pmGAME, SET AND THE MATCH JUAN MARTIN DEL POTRO
4.20pmThe second set aside, Del Potro was terrific there. He fully deserved to defeat Federer. In doing so, he has also booked his place in the semi-finals. He is likely to finish second in the group, which will mean a meeting with world number one Novak Djokovic.
4.23pm"I'm so excited to reach the semi-finals. I served really well in the last game. I was so nervous but I got it done," Del Potro tells Sky Sports News during his post-match interview.
4.25pmThat's just about all we have time for from the O2 this afternoon. Be sure to join us this evening though for coverage of David Ferrer vs. Janko Tipsarevic as the curtain comes down on ATP World Tour Finals group stages. We will be with you around 7pm. See you then.