Serena Williams is through to the final of the Bank of the West Classic after a straight-sets win over Andrea Petkovic in Stanford.

The number one seed was made to battle in the first set as the first 11 games went with serve until Williams finally made the breakthrough.

The second was a far more routine affair and Williams closed 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 will face Angelique 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 but Kerber took a tight second and the German pulled away in the decider 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."