Serena Williams has opted not to defend her title at the Bank of the West Classic.

The world number one withdrew from the Stanford-based tournament on Friday, citing an elbow injury.

That is the same issue that led to her withdrawal from the Bastad event two weeks ago after a first-round victory.

Williams has been troubled by the elbow since May.

"I'm disappointed to have to withdraw from Stanford, but I need to get my elbow back to 100 per cent," she said in a statement.

"The tournament is one of my favourites and the fans have always been so generous and supportive of me. I wish all of the players much success and I hope to return next year."

Williams, 33, won the Bank of the West the last three times she entered it, in 2011, 2012 and 2014.

But her focus now is on getting healthy ahead of the US Open, which begins on August 31.

Williams currently holds all four major titles after winning Wimbledon earlier this month and with a title in New York would become the fourth woman to complete a calendar-year Grand Slam.

Steffi Graf was the last to do it, in 1988.