Defending champion Roger Federer stormed into semi-finals of the ATP World Tour Finals Thursday by beating David Ferrer 6-4, 7-6 (5).

Federer was not in his usual rhythm, with the Swiss master having to save six break points in his first two service games.

"I definitely needed a good second serve, especially in the first set," Federer said after the match. "I didn't have that many serves in play the way I wanted to. Either the rhythm was off a bit or I wasn't pushing enough with the legs or whatever it might have been."

With this victory Federer remained undefeated against Ferrer. They have faced each other 14 times during the last 10 years and Federer has come out victorious in all occasions. Considering Ferrer's remarkable form this year, it was believed that he might register his first victory against the defending champion.

Ferrer, who won the Paris Masters last week, did not give away the match easily and presented tough competition to the 17-Grand Slam winner.

He took 0-40 lead in the opening game, but could not convert it to a win. "I had a lot of chances ... I couldn't do it," Ferrer said. "Sometimes with these players, if I don't take my chance, I don't win the important points, it's very difficult to win these matches."

Ferrer had done his homework before that match and he came with a good game plan. He kept Federer busy in long rallies, forcing the world number two to go for his shots, leading to errors.

"Ferrer showed why he's so tough to beat," Federer said. "He makes you hit the extra shot. He makes it physical.

"You know that mentally he's not going to go anywhere."

If Federer wins the ATP Finals, it would be his seventh title this year. He will face Juan Martin Del Potro in his last group match Saturday.

If Ferrer wins the tournament, then he will usurp his good friend and compatriot Rafael Nadal as the world number four. Nadal has not played a single match since Wimbledon this year due to a recurring problem with his knee.

"I don't think about that because Rafa is injured. For me, I don't care if I finish 4 or 5," Ferrer said. "I am trying to do my best to have a great tournament here. And after here, then next week I will have a very important Davis Cup final."