Roger Federer's waltz through the Australian Open takes a quick-step up in tempo when he faces by far his biggest test at Melbourne Park this year.

Big-serving Andy Roddick takes on Federer in their men's singles semi-final at Rod Laver Arena, with the Swiss master concerned at the speed and accuracy of the American's rocket-like serves in the tournament to date.

Sixth seed Roddick leads the Open's ace count with 71, including one unreturnable delivery clocked at 231 km/h.

"I think he's really found his serve again," Federer said.

"His serve kind of got lost all of a sudden. He didn't get the same free points any more. He couldn't really put the pressure on the opponent because it was too easy to return his serve.

"I don't know if it was due to change of tactics or change of conditions.

"All of a sudden, it got really slower. He's definitely picked that up again. Ever since, he's been a great player again."

Federer has not dropped a set all tournament and holds an imposing 12-1 record over Roddick.

On offer for the winner is a place in the final against either Chilean 10th seed Fernando Gonzalez, who belted world No.2 Rafael Nadal, or German 12th seed Tommy Haas.

Gonzalez and Haas play their semi-final on Friday.

Resurgent Serena Williams continues her bid for a third Australian women's singles title in her semi-final against Czech teenager Nicole Vaidisova.

Williams, who has defied fitness concerns to put herself within one match of another grand slam title, has taken all before her in the tournament so far.

Vaidisova, a French Open semi-finalist last year, was within two points of beating world No.3 Svetlana Kuznetsova to make the final at Roland-Garros but failed.

But the 17-year-old believes the experience has toughened her mentally and she is ready to take on the formidable challenge of the former world No.1.

Australian Paul Hanley will attempt to advance to the men's doubles final when he and partner Kevin Ullyett of Zimbabwe take on veteran Swede Jonas Bjorkman and Max Mirnyi in their semi.

Another Aussie, Samantha Stosur, will play her mixed doubles quarter-final at Rod Laver Arena.

Stosur and partner Leander Paes play Daniel Nestor and Elena Likhovtseva.

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