Serena reaches Stanford semis

Top seed Serena Williams fought back from a set down to book her place in the Stanford Classic semi-finals with a hard-earned 2-6 6-3 7-5 win against Ana Ivanovic on Friday.

The 32-year-old American struck back despite losing the first set to the fifth-seeded Serb to claim the second before seeing Ivanovic off by grabbing the decisive break in the deicider to take the match in just under two hours.

"It wasn't very easy," said the 17-time grand slam winner. "Ana plays very well. This surface was a little fast so she was hitting a lot of shots.

"But I just fought it and stayed in there. I just tried to do everything that I worked on."

However, there won't be an all-Williams showdown in the last-four after Serena's older sister Venus slipped to a 6-1 3-6 7-5 defeat against eighth-seeded German Andrea Petkovic.

Earlier, Angelique Kerber and Varvara Lepchenko were clinical in securing their semi-finals berths, each needing less than an hour to claim respective victories over Garbine Muguruza and Sachia Vickery.

Third seed Kerber had a testing three-setter against Coco Vandeweghe in the previous round but the German had no such concerns in the quarter-finals as she brushed aside Spaniard Muguruza 6-2 6-1 in just 57 minutes.

Kerber will now play Lepchenko, who was even more devastating in seeing off her American compatriot Sachia Vickery.

Lepchenko broke her teenage opponent's serve six times en route to a 6-1 6-0 demolition in a match that last only 52 minutes.