Maria Sharapova upset world No. 1 Victoria Azarenka in straight sets on Saturday to book an encounter with Serena Williams in Sunday's WTA Championships final.

Sharapova beat Azarenka 6-4, 6-2 after earning two early breaks in the first set and taking control in the second when Azarenka began to struggle with a problem affecting her right thigh.

"When you're playing the World No.1, you have to be ready for every shot - you have to run because the point can keep going and going," Sharapova said on the WTA website.

"I'd lost to Victoria the last few times, so I'm really happy I put myself in the semifinals to play against her and try to improve that - I was hoping to have a better result, and I'm really happy I did today."

The Belarusian leads Sharapova 7-5 in head-to-head meetings.

Despite the defeat, the Belarusian will hold the world No. 1 spot at the end of the season after winning six titles including her first Grand Slam tournament at the Australian Open.