Muguruza cruises as Sharapova pulls out

Wimbledon champion Garbine Muguruza cruised to a straight sets win over young American Kayla Day on Wednesday to make the quarter-finals of the Stanford Classic.

Muguruza dropped just two games to day as she won 6-2, 6-0 in just under an hour, a stark contrast from their previous meeting at Indian Wells when Day won the first set before finally succumbing in three.

The Spaniard's win sets up a last eight tie with number five seed Ana Konjuh of Croatia, who had earlier defeated Russian Natalia Vikhlyantseva 6-4, 6-3.

Earlier in the day, five-time Grand Slam champion Maria Sharapova had pulled out of her match with Lesia Tsurenko on the advice of doctors.

Sharapova said she felt pain in her left arm during her first round defeat of Jennifer Brady.

"Unfortunately, I have to withdraw from today's match," a statement from Sharapova read. "Toward the end of Monday night's match, I felt pain in my left forearm. After yesterday's scan, the doctor has recommended I don't risk further injury. “

Tsurenko will now play Madison Keys, who came back from a set and a break down to eventually overcome the challenge of qualifier Caroline Dolehide 3-6, 6-2, 6-3.

Keys struggled to find her range in the opener, hitting 11 unforced errors as the world number 212 put her under pressure, but it all clicked into gear for Keys after losing her serve in the first game of the second set.

"It was one of those matches where I figured it out in the middle of the set, and was able to settle in," Keys said after the win.

"It was more the kind of progression of not playing well, figuring it out, being conservative and then going for it – I'm really happy with how I did that today."