Should he hit an overhead, or let it bounce?

With the match on the line, the world's greatest player made the wrong choice, took a swing when the ball reached chest level and whacked it into the net.

Two points later, Federer had lost to his new nemesis. Guillermo Canas beat Federer for the second time this month, 7-6 (2), 2-6, 7-6 (5) in a fourth-round epic at the Sony Ericsson Open.

The upset wasn't the only rematch repeat Tuesday. Serena Williams beat Maria Sharapova 6-1, 6-1, in a rout reminiscent of their Australian Open final two months ago.

With that, Key Biscayne's two No. 1-seeded players were eliminated.

Two weeks ago, Canas ended Federer's 41-match winning streak at Indian Wells. The dogged Argentine showed that victory was no fluke, extending points by repeatedly chasing down shots and forcing a frustrated Federer into 51 unforced errors.

"It's one of those matches I never should have lost," Federer said.

The defeat ended his bid for a third successive Key Biscayne title, while Canas improved to 3-0 this year against top-10 players. He was ranked as high as No. 8 before serving a 15-month doping suspension, and he had to qualify for the Key Biscayne draw.

"I'm surprised because I beat two times the No. 1 in the world. Really, I don't know what is my secret," Canas said. "I'm just trying to enjoy the moment. For me it's like a dream."

Williams, who defeated Sharapova 6-1, 6-2 in the Australian Open final, this time won by an even more lopsided score. She bristled when asked if she anticipated such an easy match.

"Haven't you learned that I expect the best of myself?" she said. "If I'm playing well, then anything can happen. I believe there could be a similar score if I'm playing well."

While Sharapova and Federer lost, Andy Roddick won his rematch against David Ferrer. The Spaniard was an upset winner when they played at Key Biscayne a year ago, but this time the third-seeded Roddick won 7-5, 6-3 to reach the quarterfinals.

Roddick's opponent Wednesday will be No. 12 Andy Murray, who overcame two match points to beat Paul-Henri Mathieu 2-6, 7-5, 6-3. American qualifier Amer Delic lost to No. 23 Juan Ignacio Chela 6-3, 6-2.

Top-ranked Justine Henin reached the Key Biscayne semifinals for the first time by beating No. 6-seeded Nadia Petrova 7-6 (4), 7-6 (4). Henin's opponent Thursday will be No. 9 Anna Chakvetadze, who beat No. 15 Li Na 4-6, 7-5, 6-2.

With a parade of Grand Slam champions on the schedule, Canas won the biggest cheers. South Florida's large Latin population made for a festive stadium atmosphere during his match, with fans singing, waving Argentine flags and shouting "Willy!" — Canas' nickname.

"As long as they don't boo my Swiss flag, it's OK," Federer said. "This was a nice atmosphere to play."

Federer may play big points as well as anyone ever, but Canas was better at pivotal moments. The Argentine kept his cool in both tense tiebreakers, while Federer looked shaken at times and converted only four of 16 break-point chances.

The match turned with Canas serving at 0-2 in the final set. Federer had four break points, each a chance to pull away, and he failed to convert them.

The decisive moment in the second tiebreaker came with Federer serving at 4-5. Canas hit a short, high lob, and Federer handled it like a weekend hacker.

"I didn't want to let it bounce," he said. "It's a tough shot to hit. I mean, I guess at 1-0, 15-love in the first set, that's not a problem. But it was tough. It was getting into the night. It kind of made it a bit tricky, and I messed it up."

Canas then closed out the win with a 125-mph service winner. That made Federer 0-2 against Canas this year and 14-0 against everyone else.

Next month the tour moves to clay — Canas' best surface and Federer's worst. How will Federer shake the slump?

"I'm not going to play him every week, so it doesn't matter," Federer said.

Sharapova faces a similar riddle regarding Williams, who has won 24 of their 29 games this year.

"Once she gets up in the set or in the match," Sharapova said, "she seems to steamroll."

Williams, ranked No. 1 in 2002-03, has played only six tournaments in the past 18 months and has slipped to 18th. But when she hit a forehand winner on match point, she raised her index finger and shouted "No. 1" to the cheering crowd.

After shaking hands with Sharapova, a jubilant Williams raised her finger again.

"Every time I do that I win, so I'm a little superstitious," she said. "I'm working toward getting to be the best. It's an uphill battle."

Seeking her fourth Key Biscayne title, No. 13-seeded Williams will play No. 8 Nicole Vaidisova in the quarterfinals Wednesday.

Playing like the Slammin' Serena of old, Williams went for a winner on most of her shots, yet still committed only 15 unforced errors. She won the final five games of the first set and the last six games of the match.

Even after dominating the opening set, she signaled for her father and coach, Richard, to come onto the court for a consultation. His advice: Approach the second set as if it were practice, to make the most of the court time.