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 McIlroy. 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 tournament put Serena on the stadium for the night session, and she found herself in a struggle from the start.
The points were long, 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.
It was a momentous victory for the No. 4-seeded Wozniacki, whose top ranking last year drew criticism because she hasn't won a Grand Slam title.
"If people talk about me or not, I mean, to be honest, as long as I win, I'm happy," she said.
She had lost her previous three meetings against Williams.
"Serena has won so many Grand Slams and so many tournaments, something that I would love to achieve one day," Wozniacki said. "So to beat someone like her that I know that never gives up is definitely something that means a lot to me."
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.
Information from The Associated Press was used in this report.