Serena sets up Kerber final

Top seed Serena Williams will be aiming to land her third title in four years when she meets Angelique Kerber in the Stanford final on Sunday.

The American made it through after a straight-sets win over Andrea Petkovic, although she was made to battle in the first set as the first 11 games went with serve until Williams finally made the breakthrough to take it.

The second was a far more routine affair as she eventually turned what started as a closely-fought contest into a one-sided romp to close out a 7-5 6-0 win in an hour and 17 minutes.

"I'm really excited," Williams told the WTA's official website. "Andrea and I both had really long matches yesterday, so I was just hoping to come out again today with fresh legs, and just try to do the best I could out there.

"My body feels great. I'm just really excited to be in the finals of Stanford again."

Williams, winner of 60 WTA titles, is playing her first event since a surprise third round loss to Alize Cornet at Wimbledon in June.

And she will next face Kerber, who battled for over two and a half hours in coming from a set down to beat American Varvara Lepchenko.

Lepchenko broke twice to win the opening set and served for the match at 5-3 in the second set, but was broken before losing the tiebreak.

The German pulled away in the deciding set to complete a 4-6 7-6 (7-4) 6-2 win.

Kerber said afterwards: "I have no idea how I did it. I was just trying to fight until the last point and believe in myself, and just try to change my game in the middle of the match, but I have no idea how I won."