Andy Murray has blamed his troubles on serve for the defeat he suffered to Kei Nishikori on the opening day of the ATP World Tour Finals.

The 6-4 6-4 loss to the US Open finalist means that Murray now faces a battle to reach the knockout stages of the season-ending tournament, but he remains optimistic that he can produce his best form later in the week.

"I didn't serve well enough today, that was the biggest difference," the Scot told members of the media after the game.

"When you're not serving well, it's difficult to dictate the points. He dictated a lot of points behind my second serve. That's where he had the upper hand and that was the difference.

"I still have a chance, unlike other events. If this was anywhere else I would be out of the tournament. I need to work on some things tomorrow and try to improve for Tuesday."

Murray's second match will be against either Milos Raonic or Roger Federer.