By Martyn Herman

LONDON (Reuters) - With six-times champion Roger Federer absent, Swiss fans dotted around London's O2 Arena were rooting for Stan Wawrinka at the ATP World Tour Finals on Monday but left feeling short-changed after their man was thrashed by Kei Nishikori.

Wawrinka, who claimed a third grand slam title at this year's U.S. Open, should have been a decent deputy for the 17-times grand slam champion who is missing the year-ender for the first time since 2001 because of injury.

It did not look like it against Nishikori though as he sprayed 31 unforced errors in a 6-2 6-3 defeat to the Japanese who could climb above him to finish the year third in rankings.

He was uncomfortable from the start, saving break points in his opening service game, but once he dropped serve midway through the first set he offered precious little resistance.

"It was not a great match compared to what I can do, that's for sure," Wawrinka, the oldest player in the eight-man draw, told reporters. "Wasn't a great day for myself.

"I was a little bit slow on everything. I was hesitating a lot with my game, my movement."

Wawrinka has made three semi-finals in London, twice losing to compatriot Federer including a gripping duel two years ago.

If he is to reach that stage again he will have to make a substantial improvement in his remaining group matches against world number one Andy Murray and big-serving Croat Marin Cilic.

He drew comfort in the fact that he managed only five games in his first match against Rafael Nadal here last year.

"It's not the first time that I lost the first match here," Wawrinka said. "Last year I lost against Rafa, really bad match.

"I'm sure I still have something inside me to play some great tennis before the end of the year, so I'm going to try everything for that in the next match.

"I'm going to do what I need to do tomorrow to get ready for trying to play better in two days."

Wawrinka offered a few signature backhands -- the shot that has dismantled Novak Djokovic in grand slam finals -- but they were few and far between.

Nishikori got the first break in the fifth game with a punishing forehand and was ruthlessly efficient as he polished off the set with the help of another break when Wawrinka double-faulted to end a woeful service game.

A netted forehand gave Nishikori a break point in the fifth game of the second set which he converted when Wawrinka elected to leave a return that floated in.

Nishikori claimed some revenge for this year's U.S. Open semi-final loss when he broke again to end the contest after Wawrinka wafted a lazy backhand over the baseline.

(Editing by Pritha Sarkar)