Roger Federer produced a masterful performance to beat Novak Djokovic 7-5 6-2 at the ATP World Tour Finals and end the world number one's 23-match winning streak.
Defending champion Djokovic has displayed imperious form throughout 2015, reaching 14 finals in succession prior to this event, which he began with a 6-1 6-1 trouncing of Kei Nishikori.
However, the Serbian tasted a rare defeat in London on Tuesday as the ever-classy Federer - a six-time winner of the World Tour Finals - emphatically proved he is still able to take down the very best, booking a semi-final berth in the process.
After staving off an early break point, Federer gradually grew in stature and demonstrated his form by following a delightful drop shot in game six with flat winners from both wings as he moved into a 5-4 lead.
Two set points came the way of the Swiss when Djokovic twice produced unforced errors in the 12th game, and the second was accepted courtesy of a superb half-volley drop shot.
Djokovic was suddenly making increasingly frequent mistakes and found himself in deep trouble when he netted a backhand to fall 2-0 down in the second set.
Although Federer initially let his opponent back in with two loose forehands in the next game, Djokovic was soon broken again and there was to be no way back.
One match point was saved when the top seed, serving at 2-5 down, won the standout rally of the contest, but another unforced error proved costly for Djokovic, who faces Tomas Berdych next in what is now a crucial final round-robin fixture.
Federer  bt Djokovic  7-5 6-2
Federer - 6
Djokovic - 2
BREAK POINTS WON
Federer - 4/8
Djokovic - 1/3
FIRST SERVE PERCENTAGE
Federer - 66
Djokovic - 72
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Federer - 75/66
Djokovic - 51/66
Federer - 68
Djokovic - 51