WIMBLEDON, England — Novak Djokovic was upset in straight sets by Marat Safin in the second round today, ending the Serb’s chances of testing his theory about Roger Federer’s vulnerability.

The 75th-ranked Safin won 6-4, 7-6 (3), 6-2 on Centre Court, stunning the third-ranked Djokovic who was ranked third and came into Wimbledon confident after beating top-ranked Federer in the semifinals at the Australian Open en route to his first Grand Slam title.

But the 21-year-old Djokovic came up against one of the toughest second-round opponents he could have drawn.

Former No. 1 Safin has won two Grand Slam titles and also had a semifinal upset against Federer in Australia, beating the Swiss star before winning the 2005 title.

Djokovic said Federer, bidding for his sixth consecutive Wimbledon crown, was vulnerable after his recent lopsided French Open loss to No. 2-ranked Rafael Nadal.

The hype surrounding those comments set up an enticing semifinal here. Now, it will be Safin who tries to go down that path.

Djokovic seemed to be vulnerable himself, playing on a surface he is not entirely comfortable on and struggling with his serve in a blustery breeze. After saving three match points, he served a double-fault to give his Russian opponent a fourth, then double-faulted again to concede.

He shook his head as he walked forward, and hugged Safin across the net.

Grass is not Safin’s favorite surface, either. Wimbledon is the only major where he has not advanced beyond the quarterfinals. But he is a dangerous floater now.

Safin said he came in under the radar, with Djokovic under all the pressure.

“He’s the one who has to win matches. For me, nobody expects anything,” said Safin, who admitted he had not dared look beyond the second round. “Now, I’ll have to check — the way I’m playing now, I could go far.”

Federer is most at home on grass — he is on a 60-match winning streak on the surface. He had a potentially tough second-round match later Wednesday on Centre Court against Sweden’s Robin Soderling.

The only other Wimbledon champion in the men’s draw, Lleyton Hewitt, advanced to the third round with a 7-6 (4), 6-0, 6-2 win over Albert Montanes of Spain, much quicker than his opening five-setter.

Another former No. 1, Juan Carlos Ferrero, had to retire with a leg injury in the third set when he was behind 6-4, 6-4, 2-1 to 20-year-old Mischa Zverev. Ferrero reached the quarterfinals last year before losing to Federer.

Two-time champion Serena Williams was to follow Hewitt on Court 2 — the so-called ‘graveyard of champions’ for its history of upsets — against Urszula Radwanska.

Last year’s runner-up Marion Bartoli faced Tatiana Perebiynis, while 2006 champion and former No. 1-ranked Amelie Mauresmo took on Virginia Ruano Pascual in other matches.

The winner of No. 1 Ana Ivanovic vs. Nathalie Dechy was to advance to a third-round meeting with China’s Zheng Jie, a 6-2, 7-5 winner over Britain’s Elena Baltacha.

In other women’s results, 2007 quarterfinalist Nicole Vaidisova dropped nine straight games to fall behind a break in the third set before recovering to beat Australia’s Samantha Stosur 6-2, 0-6, 6-4.

She’ll play another Australian in the third round after Casey Dellacqua beat Pauline Parmentier 1-6, 6-2, 6-3.