Roger Federer beat Britain's Andy Murray in three sets on Tuesday at the ATP World Tour Finals to guarantee finishing the season as the world number one.
The Swiss won the Group A round-robin match 3-6, 6-3, 6-1 in one hour and 57 minutes before a capacity crowd of 17,500 at London's O2 Arena as the world number four slowly crumbled.
The win earned Federer enough ranking points to ensure he cannot be overtaken by Spain's Rafael Nadal at the season-ending tournament of the world's top players in the British capital.
In Thursday's final Group A games, Federer must defeat Argentina's Juan Martin del Potro - who beat the Swiss in this year's US Open final - if he does not want to risk going out of the tournament.
Murray and Spain's Fernando Verdasco face off with the winner earning the chance to claim a semi-final spot.
Murray, who has won six ATP Tour titles in the last 12 months - more than any of his rivals - went into the match with Federer boasting a 6-3 overall lead, all of those contests having taken place on hard courts like the O2 Arena.
However, Federer had won their previous clash, ending a four-game Murray winning streak by defeating the Scot 6-2, 7-6 (10/8) in the Cincinnati semi-finals in August.
French Open and Wimbledon champion Federer won the toss and chose to receive. The Swiss broke when Murray hit the net, but the Scot broke straight back, winning the second game with a passing shot after a Federer return flicked up off the net.
The next three games went with serve before Murray broke again for a 4-2 lead as Federer shot wide.
The Swiss had break point at 15-40 in the next game but Murray pulled it back to deuce with an ace and won the game with a smash. Federer then held his serve to stay in the set.
In the next game, Murray sent down two aces to give himself three set points, but a mishit and a double fault saw Federer claw it back to deuce.
However, Murray aced again for a set point advantage and won it when Federer hit a return out.
Federer took the first game of the second set to love and took Murray to deuce in the next game but the Scot held out to level the scores.
The next three games went with serve before Federer broke with a cross-court shot for a 4-2 lead.
Murray took Federer to deuce in the next game but the Swiss held out to leave the Briton serving to stay in the set.
Murray won the next game comfortably and then took the serving Federer to deuce. However, Murray hit the net to give the Swiss the set 6-3.
The first two games of the third set went with serve before Federer got himself three break points up and won the third game due to a poor shot from Murray.
Federer held serve then converted a break point at the net to go 4-1 up. The Swiss then held serve to leave Murray hanging in the match by a thread.
Murray charged into the net to fire a shot long to hand Federer match point. The Swiss challenged a baseline call but it had been correctly ruled in, taking the game to deuce.
Murray blew the advantage in the first four deuces before Federer wasted a second match point. He won it on the next deuce as Murray returned into the net.