Stanislas Wawrinka unhappy with opening day at ATP World Tour Finals

Stanislas Wawrinka has admitted that he put in a below-par performance against Kei Nishikori in his opening match at the ATP World Tour Finals.

The US Open champion succumbed to a 6-2 6-3 defeat to Nishikori in just 67 minutes, and the Swiss has acknowledged that nothing went right for him on Monday afternoon.

The 31-year-old told reporters: "It was not a great match for me compared to what I can do, that's for sure.

"It wasn't a great day for myself. In general, I think he played well, he put a lot of pressure on from the beginning.

"I don't think I found anything on the court today. I was a little bit slow on everything. I was hesitating a lot with my game, my movement."

Wawrinka will return to action on Wednesday when he faces either Andy Murray or Marin Cilic.