Serena Williams at the US Open

Serena Williams kept the family flag flying at the US Open on Thursday, rolling into the third round a day after her sister Venus withdrew, revealing she had a rare auto-immune disorder. Serena, a 13-time Grand Slam champion, completed a 6-0, 6-1 romp over Dutch qualifier Michaella Krajicek in just 77 minutes and said concern for her sister hadn't hindered her preparations. "It really wasn't that difficult, to be honest. She wants me to do the best. So now, if anything, it should motivate me more." Venus Williams said on Wednesday she had been diagnosed with Sjogren's Syndrome, a condition in which the body's immune system attacks its own healthy tissues.