Juan Martin Del Potro beat Roger Federer in a three-set thriller on Thursday to scrape into the ATP World Tour Finals semis with Federer by the tightest of margins.
The Argentine beat the Swiss world number one 6-2, 6-7 (5/7), 6-3 in two hours and six minutes at London's O2 Arena, and only just earned himself a spot in the last four at the agonising expense of Britain's Andy Murray.
Federer won the group, but with Del Potro and Murray both having won five sets to four in their three Group A matches, it went down to their games record.
The Argentine went through to the semi-finals by a margin of one, his 45 games won to 43 lost record bettering the 44-43 of Murray.
Throughout the match, ATP statisticians were furiously calculating who was going through to the semi-finals.
"It's an incredible moment, my first Masters semi-final, I've nothing to lose," said Del Potro.
Though Federer went into the match with a 6-1 overall lead on Del Potro, the Argentine pulled off a second decisive victory over Federer this season, having beaten him in the US Open final in their last clash.
Del Potro won the toss and chose to receive. The decision paid off immediately as the 21-year-old reached break point three times in the opening game, taking it on the third deuce.
The French Open and Wimbledon champion took Del Potro to deuce next game but the Argentine held serve. The world number one served the following game to love.
Del Potro held serve and then earned a break point with a forehand cross-court winner, taking the fifth game when Federer double-faulted.
The Swiss kept charging to the net next game but was beaten by Del Potro's lobs, leaving him serving to stay in the set at 5-1 down.
Federer held serve with some impressive shots, including a dash to the net that paid off.
But Del Potro -- being supported by his friend Carlos Tevez, the Argentina and Manchester City footballer -- took the set next game with two serves that Federer could not return.
The Argentine held break point in the first game of the second set. However, Federer took it to deuce with a lucky net shot that clipped the tape, then won the game.
The next eight games went with serve, incuding two service games from Federer and one from Del Potro to love.
Federer's range of deft shots and smashes had the better of Del Potro on the Swiss star's serve but the Argentine was nonetheless giving his opponent few chances to break.
Del Potro held serve at 5-4 down to stay in the set. Both players took their next service games to love, forcing the tie break.
Del Potro broke for a 3-1 lead in a duel at the net. Federer broke back with a forehand winner down the line for 3-4 behind.
Federer broke again when Del Potro dumped a simple return into the net and broke once more as a sliced shot caught the Argentine out.
At set point, Federer held serve with a cross-court forehand and celebrated staying in the match.
Del Potro won the first game of the final set to love, then Federer held serve with some cunning soft shots.
Federer won the sixth game to love, meaning that at 3-3 with the Swiss mathematically through to the semi-finals whatever the result, Del Potro would be out if he lost a game.
In the next game, Federer earned his first break points, but Del Potro kept his hopes alive, crucially winning the second deuce.
The Argentine then held three break points thanks to a double fault and took a 5-3 lead when Federer shot wide.
Del Potro then served for the match and won it with an ace to go through to the semis in thrilling fashion.