Roger Federer, chasing a 100th career title, lost to Japan`s Kei Nishikori in his opening group match at the ATP Finals. The Swiss tennis star lost in straight sets 7-6 (7-4) 6-3 to the world number nine. Federer hit 34 unforced errors and was also given a warning by the umpire for swiping a ball into the stands in the first set. The 37-year-old 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. Nishikori was the far more composed player from that moment on and broke for a 4-2 lead, before closing out the match for a first win over Federer since 2014. It was Nishikori’s third win over Federer in 10 attempts.