KEY BISCAYNE, Fla. -- Standing near the net, looking up at the ball coming almost straight down in the twilight, Roger Federer hesitated.
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 uneasiness Roger Federer seemed to manifest at times during his fourth-round loss to Guillermo Canas is nothing compared to the uneasiness fans feel when an athlete returns from a doping suspension and starts winning, writes Bonnie DeSimone. Story
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."
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 hit one serve 150 mph, a tournament record. His opponent Wednesday will be No. 12 Andy Murray, who overcame two match points to beat Paul-Henri Mathieu 2-6, 7-5, 6-3.
No. 2 Rafael Nadal defeated 18-year-old Argentine Juan Martin del Potro 6-0, 6-4. American qualifier Amer Delic lost to No. 23 Juan Ignacio Chela 6-3, 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 tiebreaks, 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 tiebreak 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.