Spain's David Ferrer stunned World No. 1 Novak Djokovic to reach the ATP World Tour semi-finals on Wednesday, outplaying the Serb 6-3, 6-1 at the O2 Arena.
Man-of-the-year Djokovic produced a lacklustre display against the world number five, crashing to defeat in one hour 15 minutes to leave his hopes of progress in doubt.
Djokovic, who edged past Tomas Berdych in his opening match, will now have to beat fellow Serb Janko Tipsarevic in his final Group A round-robin match on Friday to reach the semis.
Ferrer, the 2007 runner-up at the tournament, beat Andy Murray in straight sets in his opening match after which the Briton pulled out with a groin injury.
The Spaniard was inspired against Djokovic, taking complete control after breaking serve in the seventh game of the opening set.
Djokovic, winner of three of the year's four grand slams, made 33 unforced errors throughout the match and offered little resistance in the second set.
Earlier in the day, Berdych saved a match point on the way to a 2-6, 6-3, 7-6 defeat of alternate Tipsarevic who entered the tournament after Murray's misfortune.