Roger Federer lost his opening round-robin match at the London ATP finals tournament for the first time.
The 37-year-old Swiss is bidding to win his 100th career singles title, as well as a seventh ATP crown, and a first since 2011.
Federer had won his previous 15 opening matches at the season-ending event, but put in an error-strewn display against Kei Nishikori.
Nishikori, the world number nine from Japan, had lost his previous six matches against Federer but, after winning a first-set tie-break, held his nerve to rack up a surprise but thoroughly deserved 7-6 (7/4) 6-3 victory.
World number one Novak Djokovic opens his campaign against John Isner at the O2 Arena in London today.
Federer hit 34 unforced errors - and was given a warning by the umpire for swiping a ball into the stands in the first set - in a performance that he will be keen to forget.
He will be back in action on Tuesday when he faces Dominic Thiem, who lost to Kevin Anderson earlier on Sunday.
It is the fourth time Federer has lost his opening match at the event, but he can take some comfort from the fact that in 2007 he still went on to win the title.
Anderson made up for lost time on his ATP Finals debut with victory over Austria's Dominic Thiem.
The South African, making his first appearance at London's O2 Arena for the prestigious end-of-year tournament at the age of 32, won 6-3 7-6 (12/10) in an hour and 47 minutes.
Anderson dropped just four points on his serve in a dominant opening set while forcing seven break points against the occasionally erratic Thiem serve as he went on to win.