World number one Serena Williams has expressed her 'shock' at defending champion Roger Federer's early exit from the tournament.

The Swiss pro was knocked out in the second round to Sergiy Stakhovsky, while a hoard of other players pulled out with various injuries.

"I live and die for every match, it gives me such a thrill to win here," said Williams. "After everything yesterday I just wanted to stay on my toes and be focused. I really was shocked that Roger Federer was not able to win. Stakhovsky played excellent. It was just weird."

Williams was able to progress into the third round with a comfortable straight-sets win over Caroline Garcia.