KEY BISCAYNE, Fla. — Serena Williams kept swinging on the final point, and Caroline Wozniacki kept chasing the ball down, running from side to side in a grueling, grunt-filled exchange.

KEY BISCAYNE, Fla. — Serena Williams kept swinging on the final point, and Caroline Wozniacki kept chasing the ball down, running from side to side in a grueling, grunt-filled exchange.

A weary Williams finally hit one where her relentless opponent couldn't reach it: into the net. With that, Wozniacki had a 6-4, 6-4 victory Tuesday night, and a berth in the Key Biscayne semifinals for the first time.

"I made her run," Wozniacki said.

"Everyone I play always plays the match of the year," Williams said.

Wozniacki, a former No. 1, served well and was content to play patiently from the baseline. She extending rallies until Williams would make a mistake, and the stats told the tale: Williams hit 34 winners to 12 for Wozniacki but also committed 36 unforced errors to the Dane's 13.

"I probably played about 20 percent," Williams said. "I just made a tremendous amount of errors. There's no reason for that. I'm older and I shouldn't do that. There's no excuse. I've just got to stop that. It's silly."

Wozniacki celebrated by blowing kisses to the crowd, which included her boyfriend, golfer Rory McIlory. Her opponent in the semifinals Thursday will be No. 2 Maria Sharapova, who advanced by drubbing 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."

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.

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."

In other men's fourth-round play, No. 2 Rafael Nadal swept five consecutive games during one stretch and beat No. 16-seeded Kei Nishikori 6-4, 6-4.

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, No. 5 David Ferrer 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.