Victoria Azarenka withdraws from Madrid Open

Victoria Azarenka has pulled out of the Madrid Open due to a back problem.

The fourth seed cruised into the third round with straight-sets wins but has been struggling since suffering the injury in her first-round defeat of Laura Robson.

American qualifier Louisa Chirico has replaced Azarenka and will play Daria Gavrilova for a place in the semi-finals after the Australian upset holder and two-time champion Petra Kvitova 6-3 6-4.

Azarenka, who won back-to-back titles at Indian Wells and Miami last month, joins Roger Federer in pulling out of one of the final major tune-ups prior to the May 22 start of the French Open due to back problems.

"I tweaked my back in my first match, it was cold that day," said the Belarusian world No 5.

"In my second match, I was in a lot of discomfort. I thought a day off would help, but today during a hit it came back again.

"I just don't feel I'm able to compete today."

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Azarenka, a beaten finalist in Madrid in 2011 and 2012, is now a doubt for the Rome Masters which begins on Monday.

"I want to make a smart decision for the big picture," she added.

"I have to take it day by day and see if my body's ready. I'll hope to take tests today so we can take action."