ATP World Tour Finals 2016: Andy Murray beats Marin Cilic in London

Andy Murray began life as the world number one with a convincing victory over Croatia's Marin Cilic at the ATP World Tour Finals in London.

The Briton, who replaced Novak Djokovic of Serbia at the top of the rankings last week, won 6-3 6-2 at the O2 Arena for his 20th win in a row.

Murray and Djokovic are vying for the year-end number one spot.

Japan's Kei Nishikori beat Swiss Stan Wawrinka 6-2 6-3 in Monday's other John McEnroe Group match.

The evening session saw Murray step onto court as the world number one for the first time - and he delivered the result that most of the 17,000 spectators wanted.

"A big thank you for all of the support, it was an incredible atmosphere today," Murray said as the crowd saluted him. "I really appreciate it."

Both men misfired from the baseline in the early stages, with Murray making two winners to 14 unforced errors in the first set, but it was a similar story across the net.

The Scot, 29, took his chances when they came, breaking twice and coming through despite four double faults.

There were more signs of the form that has brought Murray such success this year as the second set unfolded.

After saving an early break point with an ace, the Wimbledon champion broke at 2-2 thanks to two mighty forehand winners, and again at 4-2 with another heavy cross-court forehand.

Murray, who has yet to win this title, put his erratic start to the match down to the pace of the court, rather than any added tension.

"I didn't feel any more nervous than usual," he said.

"It's just quite different conditions in there than what we've been practising in during the week. It's a lot slower than it has been.

"I think maybe some of the errors at the beginning were more getting used to playing in new surroundings again."

Murray will face Nishikori, live on BBC Two and Radio 5 live sports extra, on Wednesday at 14:00 GMT.

While Murray and Novak Djokovic battle for the year-end number one spot, Japan's Nishikori took a significant step towards replacing Wawrinka as the world number three with a surprisingly easy win.

"That's my goal for this week," said fifth seed Nishikori, 26.

"Try to win a lot of matches and reach number four or number three. I see the opportunity to go up the ranking."

Wawrinka, 31, was playing with strapping on his left knee and struggled to make any impression on the return, despite Nishikori making just 47% of first serves.

The Swiss, who beat Nishikori on the way to winning the US Open in September, leaked 31 errors over the course of 67 minutes.

Two break of serve allowed Nishikori to take the first set with a sharp volley after 29 minutes, and a lunging forehand return in game five provided the decisive break in the second.

Asked if he had been struggling physically, Wawrinka said: "I was feeling better on the court, so I thought I could play at a better level today.

"I was expecting a good match. Didn't happen today."

Group Ivan Lendl

14:00-17:00, BBC Two and Radio 5 live sports extra - Gael Monfils v Dominic Thiem

20:00-23:00, Radio 5 live sports extra - Milos Raonic v Novak Djokovic