Andy Murray admits that it will be difficult to reach the semifinals of the ATP World Tour Finals following his loss to Kei Nishikori in the opening match of the tournament. Murray dropped the round-robin contest in straight sets, and will meet Roger Federer and Milos Raonic in his other group matches.

"[I]t's harder to qualify when you lose your first match. That's pretty obvious," he said. "But, yeah, unlike the other events, you still have a chance to go through."

The quality of Murray's opponents, however, will make that task a big challenge. "I need to win my next two matches more than likely, and win them well if I want to go through. That's going to be tricky because Milos obviously played fantastic last week in Paris, and Roger always plays well at this event. So I'm definitely going to have to play better if I want to get through."

Murray played six straight weeks in a bid to qualify for the season-ending tournament in London, but said that did not affect him against Nishikori. But, he added, "I guess if I was to play a three-hour match which was extremely physical, then maybe I would feel fatigued in the latter stages of a match like that."

Nishikori and Federer are now 1-0 in group competition, with an 0-1 record for both Murray and Raonic.