Reigning Wimbledon champ Serena Williams has pulled out of an exhibition match to get treatment for back pain, but she said Friday that would not keep her out of the London Olympics. Williams said she needed to rest her back after her two recent titles. “I’m disappointed that I won’t be able to see my (Washington) DC fans, but can’t wait to go for gold in London,” Williams said. The American has never won an Olympic singles crown but she hopes to change that when Olympic tennis competitions begin on July 28 at Wimbledon.