Caroline Wozniacki enjoyed her first victory over Serena Williams, winning 6-4, 6-4 at the Sony Ericsson Open on Tuesday to set up a semi-final clash with Maria Sharapova.
Wozniacki had lost all of her three previous meetings with the American but was on top early, breaking in the third game of the first set.
The Dane broke at the same stage of the second set, with a brilliant forehand winner down the line, and she looked set to run away with the match as she went 4-1 up with another break.
Williams, a five-times winner of this event, broke back to trail 5-3, but Wozniacki kept her focus at the end to wrap the game up.
"I think she definitely served the best she served in her career," said Williams, who was critical of her own mistakes.
"I just made a lot of errors. I just made a tremendous amount of errors. There's no reason for that," she said. "I'm older and I shouldn't do that. There's no excuse. I've just got to stop that. It's silly."
Wozniacki said the victory would give her a major boost as she tries to work her way back to number one.
"It definitely gives you confidence. Hard work always pays off, so, you know, I just need to keep going, keep working hard, and there's always room for improvements," she said.
Second seed Sharapova eased into the semi-finals after ending her jinx against China's Li Na with a convincing 6-3, 6-0 win.
The Russian had lost her previous four meetings with Li in straight sets, including the semi-finals of last year's French Open, but needed just 68 minutes to seal victory on Tuesday.
"I came into this match having lost to her the last few times. I just really wanted to change that, so I was extremely focused and looking forward to the match," said Sharapova.
"I was just really consistent today in those conditions where, you know, it's a little gusty and blowing around and I was facing a tough opponent."
It was a scrappy start to the encounter with the first five games all being broken before Sharapova eased into her baseline game and won eight consecutive games to emerge victorious.
Venus Williams will play fifth-seeded Pole Agnieszka Radwanska on Wednesday, while top seed Victoria Azarenka faces France's Marion Bartoli.