By Martyn Herman

LONDON (Reuters) - Andy Murray's climb to the top of the world rankings took a while longer than it takes thrill-seeking tourists to reach the summit of the O2 Arena roof walk. Ten years in fact.

Which is why the 29-year-old Scot will be the man to beat next week as he attempts to win the ATP World Tour Finals for the first time to seal the year-end number one ranking.

The distinctive domed arena next to the River Thames, home of the season-ending showpiece since 2009, has never been the happiest hunting ground for Murray.

He has never made the final but knows that must change if he is to stop Djokovic from snatching back the crown.

"I've said over the last few moths I expect Novak to start playing his best tennis again," Murray, who boasts a 19-match winning streak, told reporters on Friday.

"The last couple of months he hasn't but before that it wasn't just months it was years of him playing his best tennis.

"I don't think he has too much to worry about. He's played extremely well here the last few years and I'd imagine he will have a really good tournament."

Murray holds a 405-point lead over Djokovic but with 275 Davis Cup points to come off his total, next week's tournament is effectively a decider.

What is more, Djokovic, winner of the last four ATP World Tour Finals, has been placed in the easier of the two groups.

The 12-times grand slam champion opens against Austrian debutant Dominic Thiem on Sunday and will also face big-serving Canadian Milos Raonic and another first-timer Gael Monfils.

He has never lost to any of them.

Murray, on the other hand, starts on Monday against Croatian Marin Cilic, who beat him this year in Cincinnati, before taking on and Kei Nishikori, who ended his U.S. Open hopes at the quarter-final stage, and U.S. Open champion Stan Wawrinka.

It is a tough task, but there was little evidence of vertigo on Friday as Murray spoke of life from his new vantage point looking down on his rivals.

The Scot was just going about business as usual with coach Ivan Lendl, back at courtside after watching the Scot's unstoppable late-season charge from afar.

"It feels good obviously," he said of his new status. "But off court I don't feel any different this week than the last.

"When you step on the court you feel a little bit better and have little more confidence and feel better about myself.

"(Lendl) and I spoke on the Saturday. He congratulated me on getting to number one. But when we got here it was back to working on the things we are trying to improve."

Murray's rise to number one has coincided with Djokovic's dip in form, having dominated the first half of the year when he won the Australian Open and completed his career slam by winning the French Open, downing Murray in the final on both occasions.

He has failed to win a title since claiming the Rogers Cup in July and suffered a first career loss to Cilic in Paris last week, opening the door for Murray to climb to the top. But he was quick to rebuff those who say he has lost his clinical edge.

"I thought I had a good couple of months. It's not been up to the standards of the 12 to 15 months before that, but in sport you can't always expect to win," Djokovic said.

"I'm here in London to crown this year with the best possible result."

(Editing by Pritha Sarkar)