Roger Federer admits he is still struggling to figure out how to beat Andy Murray as they prepare to meet in the semi-finals of the Indian Wells Masters tomorrow.

The British No1 has beaten the Swiss in five of their seven matches, including their last three meetings.

Although Federer did win the big one against Murray - last year's US Open Final - he thinks that the 21-year-old is still something of an unknown quantity.

"He's a great player and great players are tough to play against," said Federer, 27.

"He's young, so you still have to figure him out a little bit. He's changing his game as time goes by. Every time you play him, he plays a bit different.

"Whereas for me, it's different. He knows what to expect. That's the advantage of a youngster. There are disadvantages in that they're a bit more inconsistent but he's been very consistent, at a young age, which is impressive to see."

Although Murray (right) has the advantage head-to-head, Federer takes comfort from his 13 Grand Slam triumphs.

"I'm aware he has beaten me more than I've beaten him but, on big occasions, I think I came through," said Federer.

"I have to build on that and make sure I play a tough and good match against him. Anything else is not going to do the job."

Since beating Murray in New York, Federer has lost to him in Madrid, Shanghai and Doha.

"Every time I won the first set, he kind of got stronger and stronger as the match went on," Federer said.

"I've just got to make sure I finish because I know it's not physical, the end of the matches. Three sets, it never really becomes physical. It's only maybe in a fifth set that it could happen."

Asked if he had unfinished business to resolve against Murray, the world No4 replied: "Not really."

Federer set up tomorrow's clash with a 6-3, 7-6 victory over Spain's Fernando Verdasco.

Murray, who had a hard-fought 75, 7-6 victory over Croatia's Ivan Ljubicic, is looking forward to the semi-final.

"We know each others' games pretty well," said Murray.

"If I want to beat him, I need to play one of my best matches.

"It's important to serve solid, make a lot of balls and not feel like you have to do anything special on each point to win against him."