Kei Nishikori produced a huge upset at the ATP Finals on Sunday with a straight-sets victory over a below-par Roger Federer.

Nishikori came into his opening match of the round-robin stage having not beaten Federer in their six previous meetings, but the world number nine battled to a 7-6 (7-4) 6-3 win at the O2 Arena.

It adds to a remarkable second half of 2018 for Nishikori, who started the year playing ATP Challenger events as he came back from wrist surgery.

Inflicting a rare defeat on Federer is undoubtedly the highlight of the year for Japanese, the six-time winner of the end-of-season event well short of his usual standards as he began his bid for a 100th career crown.