(Adds Djokovic quotes)
LONDON Nov 5 (Reuters) - U.S. Open champion Andy Murray withstood an early onslaught to beat Tomas Berdych in his first match on home soil since winning his maiden grand slam title and Novak Djokovic overcame Jo-Wilfried Tsonga as the ATP World Tour Finals began on Monday.
Murray, greeted on court at the cavernous O2 Arena like a returning hero less than two months after beating Djokovic in Flushing Meadows epic to end Britain's 76-year wait for a male grand slam winnner, wore down Berdych to win 3-6 6-3 6-4.
World number one Djokovic also had to contend with some blistering firepower from free-wheeling Frenchman Tsonga before registering a 7-6 6-3 victory to move top of Group A.
Djokovic and Murray will face each other in the latest installment of their rivalry on Wednesday when the winner will take a huge stride towards the semi-finals of the year-ender.
Neither had it all their own way with Murray admitting he had struggled to deal with Berdych's power.
"Some of the time in the first set he just completely overpowered me," Murray told reporters.
The Scot got a rapturous reception as he walked on court but the crowd became subdued as Berdych took the opening set and threatened to seize control of the second before Murray responded to turn the match around.
Murray's inability to convert the numerous break points he engineered cost him the first set.
The Olympic champion was 15-40 ahead on Berdych's serve in the opening game and was 0-40 up in fifth but was frustrated as Berdych produced brutal attacking tennis to stay on level terms.
A spurt of 13 points out of 15 midway through the first set, including a crucial service break in the sixth game when Murray yanked a forehand low into the net, enabled fifth seed Berdych to grab the opener in 45 minutes.
Things reached crisis point for Murray at 1-1, 15-40 in the second set but his opponent let him off the hook.
At 30-40 Murray looped in a sluggish second serve and Berdych's eyes lit up as he lined up to club away an inviting forehand. The 27-year-old Czech could hardly believe it when he missed the court by the width of a ball.
"The match slipped out a little bit (after that)," Berdych, beaten by Murray in the U.S. Open semi-finals, told reporters.
"That was the biggest moment. That was a pretty good chance but it didn't happen."
Murray carved out three more break points in the following game but again Berdych recovered from 0-40, only to gift Murray the initiative with two forehand errors.
"Both of us I thought served pretty well and weren't losing too many points on our first serve, so it can hinge on a couple of points here or there," said Murray, who needed 11 break points before finally converting one.
"That definitely gave me the advantage in the second set. But the third set was tight as well."
With the near 17,000 crowd coming to life and Murray firing himself up, the Scot levelled the match and gained a decisive break of serve in the third game of the decider on his way to only his fourth in eight matches against Berdych.
Djokovic admitted he was second-best during the first set against Tsonga when he saved break points at 3-3 and 5-5 and never threatened the Frenchman's delivery.
However, the Serb, guaranteed to end the year as No.1 for the second time in a row, showed his usual doggedness to hang on before an immaculate tiebreak sealed the opening set and Tsonga's resistance quickly crumbled.
"To be honest, he was the better player throughout the first set," Djokovic told reporters. "I managed to stay mentally tough and fight my way through."
Defending champion Roger Federer begins his campaign on Tuesday against world number nine Janko Tipsarevic, who qualified for the eight-man finale in the absence of the injured Rafael Nadal. (Editing by Ed Osmond)