Serena acknowledges the crowd after pulling out

The Australian Open champion was trailing 5-3 to Taiwan's Chan Yung-Jan when a groin injury ended her chances.

"I've been playing a lot on the clay. I was just sliding a lot and I think I just slid too far," she said.

Earlier, Venus, who won the tournament in 2004, beat Samantha Stosur 6-2 6-2 in their first-round match.

The pair had been on course to meet in the third round, which would have been their first encounter since the 2005 US Open.

Serena had a first-round bye, while Venus now faces Russian teenager Vasilisa Bardina in round two.

Elsewhere, top seed Nicole Vaidisova crashed out with a 4-6 6-4 6-3 second-round defeat to wildcard Michaella Krajicek.

"I didn't feel in any part of the match I played well," said Vaidisova after the defeat.

"I wasn't really hung up on the fact I was the number one seed, but of course I'm disappointed."

Tatiana Golovin, who won her first tour title at Amelia Island at the weekend, kept up her recent good form with a 6-4 6-1 win over Dominika Cibulkova.