Stan Wawrinka's hopes of reaching a fourth consecutive semi-final at the ATP World Tour Finals were dented by an opening loss to Kei Nishikori.

The third seed, who sported a poppy on his shirt, had never previously lost to anyone at the O2 Arena other than Roger Federer, Rafael Nadal or Novak Djokovic.

But he was well below his best and fifth seed Nishikori ran out a convincing 6-2 6-3 winner.

The victory boosts Nishikori's hopes of overtaking Wawrinka to finish the year as world No.3, and left the Japanese star satisfied.

"That's my goal for this week, try to win matches and try to reach No.4 or No.3," Nishikori said.

"I've been playing really well this year so I'll try to keep it up. Today I think I was very solid from the first game and I was very confident. I thought I played a good match."

Nishikori next faces the winner of Tuesday's match between world No.1 Andy Murray and Marin Cilic.

Wawrinka led their head-to-head 4-2, winning their most recent meeting in the semi-finals of the US Open before going on to win his third grand slam title.

But, brilliant as he can be, the Swiss is also erratic and there were far too many errors in the opening set in particular.

Wawrinka tried to fire himself up at the start of the second set and initially played better, but he then misjudged a return that landed on the baseline and was broken in the fifth game.

The 31-year-old never looked like recovering and, when he drilled a backhand long after an hour and seven minutes, the match was over.