Top-ranked Victoria Azarenka overcame a shaky start to beat Li Na 3-6, 6-3, 6-3 and advance to the Madrid Open semifinals on Friday.
In the second quarter-final, Serena Williams breezed past Maria Sharapova 6-1, 6-3 to reach the semifinals.
Azarenka struggled on serve early on, but last year's runner-up found her stride on the new blue clay-court and broke her Chinese opponent's first two service games of the second set to pull even.
Azarenka then used her powerful ground strokes to jump out to a 4-0 lead in the decider before holding off Li's late charge.
The Belarusian will face the winner of the quarterfinal between Agnieszka Radwanska and Varvara Lepchenko. Azarenka has won four titles this season.
Also, Agnieszka Radwanska plays Varvara Lepchenko, while Sam Stosur faces Lucie Hradecka.