Taking the court as the No. 1-ranked player for the first time, Andy Murray got the job done. The Wimbledon champion was tested for much of his opening match at the ATP finals but still came out on top, playing in his home country and beating Marin Cilic 6-3, 6-2 Monday at the O2 Arena.

"It was obviously nice to play in that stadium with the crowd like that. It obviously helps," Murray said. "After a long, kind of, few months, it's nice to know that I'm going to be finishing the year playing in that sort of atmosphere."

Murray took over at the top of the rankings last week, replacing Novak Djokovic. But the year-end No. 1 ranking is still up for grabs and could be decided on Sunday if both Murray and Djokovic reach the final. The prospect of attaining that, however, doesn't seem to be weighing too heavily on Murray's mind.

"The last few days, I haven't thought about it too much," Murray said. "It didn't change much for me."

It certainly hasn't affected his ability to win matches. Murray has won a career-high eight titles this season, including a second Wimbledon title in July and a second straight Olympic gold medal in August. Since then, he has won titles in Beijing, Shanghai, Vienna and Paris.

The latest victory on Monday didn't come so easy. Murray struggled to win his service games in the first set against Cilic, but still ended up saving all but one of the five break points he faced in the match. On the other side, he converted four of his five break points.

"When I need to step in, I didn't. When I need to wait a little bit longer, I wasn't waiting," said Cilic, the 2014 U.S. Open champion. "The hitting could have been better. Yeah, a little bit unfortunate to go 6-3, 6-2 down, where it felt it was much, much closer in the first set. Beginning of the second, I had couple chances as well."

In the early match, Stan Wawrinka finally lost a match at the ATP finals to someone who hasn't been ranked No. 1 in the world. The U.S. Open champion, who has reached the semifinals of the season-ending tournament for the last three years, was beaten by Kei Nishikori 6-2, 6-3.

"It was not a great match compared to what I can do, that's for sure," said Wawrinka, a three-time Grand Slam champion. "I don't think I find anything on the court today. I was a little bit slow on everything. I was hesitating a lot with my game, my movement."Wawrinka holds a 6-7 record at the tournament for the top eight players in the world, but his previous six losses were against Djokovic, Roger Federer and Rafael Nadal - twice to each.

Monday's loss doesn't mean Wawrinka is out of the tournament, but it hurts his chances of finishing the year as the No. 3-ranked player. Wawrinka, Nishikori and Milos Raonic all have a chance to end up one spot behind Murray and Djokovic.