Andy Murray vowed to come back fighting against Milos Raonic today as he bids to keep alive his ATP World Tour Finals campaign.

The Scot's hopes of progressing to the semi-finals were dealt a serious but not quite terminal blow on the first day of play at the O2 Arena with a 6-4 6-4 loss to debutant Kei Nishikori.

Roger Federer completed the opening matches in Group B with a 6-1 7-6 (7/0) victory over Raonic to set up a meeting with Nishikori this afternoon.

The glimmer of light for Murray was that Raonic also failed to win a set, and a straight-sets victory in their match would at least give the British number one a fighting chance.

He would still probably need to beat Federer to progress, but Murray certainly is not giving up just yet.

"It's harder to qualify when you lose your first match, that's pretty obvious," he said. "But, unlike the other events, you still have a chance to go through. If this were anywhere else, I would be out of the tournament.

"I need to try to forget about today, work on some things tomorrow, and hopefully play better on Tuesday."

Murray played six weeks in a row, winning three tournaments, to book his spot at the eight-man event in London, but there was no sign of that form in a passive and disappointing performance.

He said: "You're competing against the best players in the world. And Kei is obviously playing well just now, so I knew that this was a possibility.

"But it's still obviously disappointing to lose the first match. And I would have liked to have done better.

"I didn't serve well enough. I would say that was the biggest difference in the match. He was able to dictate a lot of points, especially behind my second serve."

Stan Wawrinka put a poor run of form behind him in spectacular fashion as he opened his account by thrashing Tomas Berdych.

Wawrinka had won only one match since the US Open as a season that began with his stunning Australian Open triumph threatened to end with a whimper.

But he raced out of the blocks and an off-key Berdych had no answer, with third seed Wawrinka winning 6-1 6-1 in only 58 minutes.

It was the most one-sided match since the tournament moved to London in 2009.

The result maintains Berdych's unwanted record of never having won his opening match at the tournament, and he failed to hit a single winner off the ground.