New York- Serena Williams and older sister Venus have accepted a doubles wildcard for the U.S. Open, tournament organisers said on Saturday. The Williams sisters are two-times doubles champions at season’s final major, having triumphed together in 1999 and 2009. Together they have won 14 women’s Grand Slam doubles titles. The US Open, which begins on Monday, could be the final tournament of Serena’s glittering career as she announced last month her pending retirement but never confirmed Aug. 29-Sept. 11 as her final event. Serena, 40, and Venus, 42, are also competing in the singles draw.