Serena Williams has some work to do to get in shape for her next season on the court. Guests at the launch of the Louis Vuitton 2010 Cruise collection at Saks Fifth Avenue marveled at how pretty the tennis great looked in her black dress, but also gawked at how big her behind was. Maggie Gyllenhaal also brightened up the bash with Eric Lucas, Kimberly Guilfoyle, Milly de Cabrol and Amy Sacco.