By Caroline Cheese BBC Sport at Wimbledon

Federer (left) has lost to Safin twice

Roger Federer could face the first test of his title credentials when he faces unpredictable Russian Marat Safin in the third round of Wimbledon on Friday.

The clash between the former world number one and the current incumbent is the third match on Centre Court.

Two-time finalist Andy Roddick and Fernando Gonzalez are also in action on day five, along with women's contenders Serena Williams and Justine Henin.

However, rain is once again set to upset the schedule in SW19.

"Hopefully we'll get it through," said Federer, seeking his fifth straight title at the All England Club.

"I expect a tough match. I played him on grass in Halle last year and I had a difficult three-setter against him.

I have nothing to lose. I need to take my chances - because there are going to be chances Marat Safin

"He's a player who can cause an upset against anybody on any day. I hope he's not going to have one of those crazy-good days against me.

"But I think I can pretty much control how things are being played on grass."

Safin has beaten Federer twice - including at the Australian Open in the 2005 semi-finals - but the Russian's least favourite surface is grass and he has never got beyond the last eight.

"It's a huge challenge," said the 27-year-old former US and Australian Open champion.

"If I want to have a chance to beat him, I have to put up my best game, go for it.

"I have nothing to lose. I need to play my tennis, stay focused and take my chances - because there are going to be chances."

606: DEBATE Will Safin cause an upset?

Roddick, who is seeded to meet Federer in the semi-finals, is up against big-hitting Spaniard Fernando Verdasco in the third match on Court One.

Gonzalez, the Australian Open finalist, opens play on Centre Court against Serbian Janko Tipsarevic.

In the women's event, Serena will be expected to cruise past Venezuela's Milagros Sequera.