New world No.1 Andy Murray began his campaign at the ATP World Tour Finals with an impressive 6-3 6-2 defeat of Croatian powerhouse Marin Cilic at the O2 Arena in London on Monday.

Murray was greeted like a returning hero by the home crowd as he strode on to the blue court for his first match since officially taking top spot and he did not let them down.

He was pushed hard though by Cilic who matched Murray for most of the encounter without making the most of his chances.

Murray fought off a succession of break points in the opening set and although he did drop his serve once, he replied with two breaks of the Cilic serve to take the set in 45 minutes.

The battle-hardened Scot broke in the fifth game of the second set with a belting forehand and again two games later as Cilic ran out of ideas.

Murray wrapped up a 20th consecutive victory with a flourish to join Japan’s Kei Nishikori at the top of the John McEnroe group after he beat Stan Wawrinka earlier.

The 29-year-old needs to outperform Serbia’s Novak Djokovic, winner for the past four years at the London year-ender, to be sure of finishing the year ranked one.

Meanwhile, Stan Wawrinka’s hopes of reaching a fourth consecutive semi-final at the Tour Finals were dented by an opening loss to Kei Nishikori.

The third seed, who sported a poppy on his shirt, had never previously lost to anyone at the O2 Arena other than Roger Federer, Rafael Nadal or Novak Djokovic.

But he was well below his best and fifth seed Nishikori ran out a convincing 6-2 6-3 winner.

The victory boosts Nishikori’s hopes of overtaking Wawrinka to finish the year as world No.3, and left the Japanese star satisfied.

“That’s my goal for this week, try to win matches and try to reach No.4 or No.3,” Nishikori said.

“I’ve been playing really well this year so I’ll try to keep it up. Today I think I was very solid from the first game and I was very confident. I thought I played a good match.”