Surprise packet Serena Williams has dropped an intriguing Twitter bombshell, with the former No 1 hinting that she might be teaming up with Andy Roddick for some mixed doubles in Australia. The possibilities for that to happen are limited, with the best guess the Hopman Cup mixed team event starting December 31. Williams won the trophy for the US in 2003 with James Blake and four years later with Mardy Fish. Her post-US Open tweet, however, begs as many questions as it answers. "Me and @andyroddick decided