Kickserv, although Clijsters choked that's what happens to most players when they have a winning opportunity against the world's #1.
Not only did Clijisters choke, but Serena did her best to present the match to her opponent.
In a total of 31 games for the entire match, Clijsters hit only 14 outright winners, less than one winner every two games. Compare this to Serena's 65 un-forced errors (32 more than Clijsters), that's over two points per game and you start to get the picture.
The better player won, by giving up so many cheap winners Serena showed she is a champion without bringing her "A" game to the court.