Top seed Victoria Azarenka saw off China's Li Na with a battling 3-6, 6-3, 6-3 win to reach the semifinals of the Madrid Masters on Friday.
The Belarusian world number one will now play either Agnieszka Radwanska, the Polish fourth seed, or American qualifier Varvara Lepchenko.
Azarenka, January's Australian Open champion, levelled her head-to-read record against French Open champion Li at 4-4 with the victory.
Eighth seed Li was playing in her second clay quarterfinal after Stuttgart.
Ninth seed Serena Williams won a battle of former number one players as she defeated second seed Maria Sharapova, the Stuttgart champion, 6-1, 6-3 with 11 aces, 28 winners and five breaks of serve in 80 minutes.
Williams said that while the blue clay courts remain problematic, the issue was not enough to make her consider skipping the event in 2012 as Rafael Nadal and Novak Djokovic have threatened to do.
"This is a tough surface and it's extremely slippery," said the 13-time Grand Slam-winner.
"But Djokovic is still in the event and so is Roger (Federer), last time I checked. If I'm not here next year, it won't be because of the clay.
"I don't think there's been any improvement in the courts over the week, but every clay court is different. This is not the best court -- definitely not what they play like at Roland Garros.
"But it's what they use at Madrid. Maybe next year they'll put more clay on it. When it's hot, it gets more slippery. But that's an element you cannot control and it goes for every clay court."
Williams is unbeaten against Sharapova since 2005 and is riding a 10-match winning streak, her longest in a year.
Azarenka said that after losing the opening set against Li, she still has no idea how she was able to turn the match around for victory in just under two hours.
"She played unbelievable in the first set," said the top seed.
"I knew I had to do something. I don't know if it was confidence or self-belief or what. I don't know what happened but I managed I started to get into the match and build up my game."
The Belarusian said that despite this week's controversy over the slickness of the new blue courts, the subject is not worth discussing until the tournament is over.
"I see no reason to talk about the courts until after the event. These are the conditions we have to deal with."