KEY BISCAYNE, Fla. (AP) — Serena Williams' bid for a record sixth Key Biscayne title ended Tuesday night when she lost in the quarterfinals of the Sony Ericsson Open to former No. 1 Caroline Wozniacki, 6-4, 6-4.

Wozniacki served well and was content to keep the ball in play from the baseline, using her retrieving skills to extend rallies until Williams would make a mistake. Williams hit 34 winners to 12 for Wozniacki, but also committed 36 unforced errors to the Dane's 13.

Williams, seeded 10th, was playing in her first tournament since January after being sidelined by a left ankle injury. She hasn't won a title since August.

The buzz Andy Roddick got from beating Roger Federer turned into a hangover when Juan Monaco became a real headache. Less than 24 hours after his upset win over Federer, Roddick endured a drubbing against Monaco 7-5, 6-0.

Monaco, a patient baseliner seeded 21st, was unfazed by Roddick's big serve, and the match became a succession of long rallies. Roddick began to look weary as the match progressed and stumbled after several shots as the match slipped away.

Roddick said he's not in peak condition after being sidelined by right hamstring and right ankle injuries earlier this year.

"I just didn't have it physically," he said. "I got to about 4-all, and I was — you know, I'm out of shape. That's it."

The loss was a big comedown after Roddick beat Federer for only the third time in 24 tries, temporarily silencing critics who say he should consider retirement.

Roddick's ranking has slipped to 34th, his lowest since 2001. He might return to the top 30 next week, and he said his game is headed in the right direction entering the clay-court season.

"My tennis has come around a long way in the last two weeks, maybe three weeks," he said. "Now I feel good enough where I feel like I can put in the work away from the court and get my legs back under me as far as strength and fitness."

No. 2-seeded Maria Sharapova became the first semifinalist when she routed reigning French Open champion Li Na 6-3, 6-0. Sharapova had lost their previous four meetings.

"I just really wanted to change that," she said, "so I was extremely focused and, you know, looking forward to the match."

Sharapova, who will face the No. 4-seeded Wozniacki on Thursday, seeks her first Key Biscayne title. She lost the final in 2005, 2006 and last year.

In other men's fourth-round play, No. 2 Rafael Nadal swept five consecutive games midway through his match and beat No. 16-seeded Kei Nishikori 6-4, 6-4.

The victory was by far Nadal's most taxing of the tournament, and he took a medical timeout late in the opening set for treatment of his troublesome left knee. Even so, he's one round closer to his first Key Biscayne title.

"I am not probably in perfect condition today with the left knee," he said. "But the important thing is to try to win as many matches as possible. For me this is an important tournament, and every victory has very, very big value for me, especially without being perfect."

Nadal, a three-time runner-up at Key Biscayne, lost the final last year to Novak Djokovic, and they could meet again Sunday. The top-ranked Djokovic reached the final eight by beating No. 17-seeded Richard Gasquet 7-5, 6-3.

No. 4 Andy Murray and No. 6 Jo-Wilfried Tsonga also won in straight sets.

No. 8 Mardy Fish edged No. 12 Nicolas Almagro 6-3, 6-7 (2), 6-3. Fish was relegated to the grandstand court for the third consecutive match, even though he's the top-ranked American man, but the slight didn't faze him.

"I certainly want to play on stadium court," Fish said. "But they've got to put someone out on the grandstand, and who's that going to be? Are you going to put Federer out there, or Djokovic, or Nadal, or Serena? I'm not going to go ahead of those guys as far as people coming to watch them play. You've got to go where you've got to go."

The tournament did indeed put Serena on the stadium for the night session, and she found herself in a battle from the start.

The points became grueling, with Williams unable to overpower her opponent for a change. She matched a career high with 20 aces in the fourth round but had only four against Wozniacki and lost serve three times.

Wozniacki, seeded fourth, advanced to the Key Biscayne semifinals for the first time, and she has a shot at her first final berth of the year. She had lost her previous three meetings against Williams.

There's still one Williams in the tournament: Venus plays No. 5-seeded Agnieszka Radwanska in the quarterfinals Wednesday.