Clijsters's rivalry with Serena included two of the biggest controversies in Serena's career: the 2001 Indian Wells final which led to both Williams sisters' long boycott of the tournament, and the 2009 US Open semifinal which Serena lost on a point penalty. Serena defeated Clijsters in their first five meetings. She also won all four of their three-set encounters, including their 1999 US Open third round match and their 2003 Australian Open semifinal where Clijsters had the opportunity to serve for both matches. Although Clijsters only had two wins against Serena, both were considered big upsets and among the most important wins of her career. Her victory in the 2002 WTA Tour Championship final gave Clijsters her biggest title at the time, and her win at the 2009 US Open set up her first Grand Slam singles title as a mother one match later. Clijsters also defeated Venus in both of those tournaments, making her the only player to record wins over both Williams sisters at the same event twice.