U.S. Open and Olympic champion Andy Murray entered the semi-finals of the ATP World Tour Finals with a straight sets win over Jo-Wilfried Tsonga Friday.

Earlier, Novak Djokovic secured top spot in Group A with another straight set 6-2, 7-6 victory over Tomas Berdych.

After Djokovic's win, Murray was required to win only one set to join the Serbian world number one in the semi-finals. But on Coach Ivan Lendl's advice he concentrated on defeating Tsonga and won the match 6-2, 7-6 (3).

"I didn't know exactly what I needed to do. But obviously when he said that, I kind of realized," Murray said. "Then I just asked the umpire at the change of ends after the first set whether I was through or not. Then he said I was. That was it."

He broke Tsonga's serve twice in the first set and took a 4-0 lead. He broke Tsonga's serve again early in the second set, but the Frenchman fought back to take the score to 3-3, before Murray prevailed in the tie break.

"I'm somebody who never gives up," Tsonga said. "So sometimes ... you're not playing a good tennis, and you keep fighting, you keep believing in you, and then it comes better and better, and that's it."

Roger Federer had already qualified for the semi-finals. If he manages to clinch top spot in Group B, then Murray will have to play against him Sunday.

"I think over the years, I got used to playing those guys a couple of times in a week or whatever," said Murray, who lost to Djokovic in the group stage. "It's not something that I'm afraid of. I know it's incredibly difficult and challenging to beat guys like that back-to-back, but it's doable."

After losing in the second round of the Paris Masters, Djokovic had said he was feeling a bit deflated physically. But in the match against Berdych, he looked rather fit and there were no signs of tiredness. His backhand and forehand returns were accurate, his serves were powerful and he was running all over the court to return the ball to Berdych.

"If you are winning matches, I mean, obviously you feel better," Djokovic said. "But still, I mean, the fact is that it being the last tournament of the year here in London for most of the players, it is a challenge finding that last drop of energy, physical, mental, to find their best game and go as far as they can."

Berdych lost the first set 6-2, but he played better in the second set to force a the tiebreaker, which was won by Djokovic.

"I had some chances in the tiebreak," Berdych said. "If I would be able to make it, try to push him for the third set, then maybe get another chance. But this would be just if. It didn't happen."