MELBOURNE, Australia -- The first sign or two of weakness in Roger Federer -- practically invisible to the naked eye -- is detectable by that vaunted Serbian eyesight.

Or at least with the help of Janko Tipsarevic's hip glasses.
The original seed of doubt was planted by No. 3 Novak Djokovic, who not only beat Federer in the summer in the Montreal final, but appeared to slightly irritate the world No. 1 as time went on.

Djokovic, who beat Lleyton Hewitt in the fourth round on Monday, was asked about the difference in approach to Federer, especially in the aftermath of Federer's five-set marathon in the third round against Tipsarevic.

"Well, yes. The players start to feel that he's beatable," Djokovic said. "Of course, nobody's unbeatable. But he was very, very dominant, especially on the faster surfaces. You know, now he's been No. 1 player of the world for a while. The players started playing in a different way against him, with more belief that they can win. So it's a good thing for all of us."

Still, Djokovic was not looking ahead to a semifinal against Federer. He will play No. 5 David Ferrer of Spain on Wednesday in the quarterfinals, and Federer faces No. 12 James Blake.

Federer is 7-0 against Blake and has lost one set to him, at the U.S. Open in the quarterfinals in 2006. Also on Wednesday are the remaining women's quarterfinals between No. 8 Venus Williams and No. 4 Ana Ivanovic of Serbia and No. 9 Daniela Hantuchova of Slovakia and Agnieszka Radwanska of Poland.

Ferrer is 3-2 against Djokovic, and they've split their last two meetings.

"He finished his year big time in Shanghai, so he's very motivated now," Djokovic said. "Again, he doesn't have anything to lose here. So he's been playing some amazing tennis. It's going to be a tough match for me, so we'll see what happens."

The first player to reach the men's semifinals here was No. 2 Rafael Nadal of Spain, who beat Jarkko Nieminen of Finland, 7-5, 6-3, 6-1.

Nadal will play either Jo-Wilfried Tsonga of France or No. 14 Mikhail Youzhny of Russia. Nadal has not dropped a set in five matches.

By the numbers

For all of Federer's supposed vulnerability, it should be remembered that after his five-setter in the third round, he won his next one in straight sets.

Around here, that hasn't been an easy thing to do.

For example:

--Philipp Kohlschreiber. Third-round, five-set winner against Andy Roddick. Fourth-round, four-set loser to Nieminen.

--Hewitt. Third-round, five-set winner against Marcos Baghdatis, finishing at about 4:30 in the morning here. Fourth round, three-set loser to Djokovic.

--Vince Spadea. Second-round, five-set winner, going 9-7 in the decider. Third-round, three-set loser to Ferrer.

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