LONDON, July 4 : Serena Williams pulled out of her Wimbledon doubles campaign alongside sister Venus after sustaining a knee injury during her opening-round singles defeat, stripping the Saturday draw of a major headline act on American Independence Day.Serena announced the news on social media alongside a video of herself limping with strapping on her right leg, and pictures of large syringes of fluid drained from her knee."I'm heartbroken to have to withdraw from doubles. Coming back to compete again has been a gift, and the opportunity to play alongside @venuswilliams once more meant the world to me," the 44-year-old said on Instagram."I did everything I could but unfortunately my knee just isn't ready to compete. "The photo of the syringes is the fluid they drained from my knee after m