Roger Federer ended Novak Djokovic's 23-match winning streak with a 7-5, 6-2 defeat of the Serb to book his semi-final berth at the ATP World Tour Finals in London.A single break of serve decided a cat-and-mouse opening set, Federer producing a sensational half-volley drop shot in the 12th game having put the Djokovic serve under serious pressure for the first time in the match.
With his Swiss fan club cheering on their man in a packed Millennium Dome, the 34-year-old 17-time major champion pulled ahead in the second set with an early break, only to gift Djokovic the break-back with a forehand error.
But world number one Djokovic was subdued throughout and failed to find the extra gear that he usually locates as Federer took advantage to move into a 5-2 lead.
Djokovic saved one match point after prevailing in a long baseline exchange but there was no escape a few points later when the Serb sent a forehand just wide to end their 43rd career meeting after one hour and 17 minutes.
Djokovic, champion for the last three years at the Tour Finals had not been beaten indoors since 2012, a winning streak of 38 matches.
Federer has two wins already in the Stan Smith Group with Djokovic and Kei Nishikori, who was well beaten by the Serb on Sunday, on one apiece.
Djokovic will need to beat winless Tomas Berdych in straight sets to guarantee a semi-final place.
Nishikori shook off the drubbing by Djokovic to beat Berdych 7-5, 3-6, 6-3.
The Japanese world number eight looked in trouble when he faced a break point at 3-3 in the deciding set but he finished fast to register his first win at the event.
"I got a bit lucky in the last couple of games but I worked really hard today," the 25-year-old, who reached the semi-finals on his debut at the tournament last season, said on court.
A match full of powerful baseline rallies ebbed and flowed.
World number six Berdych, who was beaten by Federer in his opening match, squandered a great chance in the first set when Nishikori was serving at 4-5, 15-30, missing a wide-open court with a backhand at the end of a brilliant rally.
It proved costly as Berdych dropped serve in the next game and Nishikori clinched the set on serve with a pinpoint forehand winner past the wrong-footed Czech.
Nishikori went 2-0 ahead in the second set but Berdych reeled off five games in a row with some thunderous winners to turn the match around and send it towards a deciding set - the first so far in the singles this year at the finals in London.
An engrossing third set reached a crucial juncture at 3-3 when a lucky net cord gave Berdych a break point, but Nishikori showed great defensive skills to fend off a big attack from his Czech opponent and kept his nose ahead at 4-3.
Two forehand errors from the Berdych racquet allowed Nishikori to move 5-3 ahead and the Japanese player secured victory with a love hold.