Serena Williams posted her fourth straight win over Maria Sharapova - but not without a struggle at the Family Circle Cup in South Carolina.

The American, who had dropped a total of five games in her previous two matches against Sharapova, triumphed 7-5 4-6 6-1 to reach the semi-final.

Sharapova served for the first set at 5-4 and Williams admitted: "I was lucky to hang in there.

"But in the third set I finally played the Serena game. I'm not new to this."

Williams is seeking her third straight title win after picking up the trophies in Bangalore and Miami.

"This is definitely a big win, but I still want to win the next round and win the tournament," said the world number nine.

Like Williams, Sharapova has been in sensational form this season, picking up three titles including the Australian Open.

However, the Russian is rarely at her best on clay and was facing her nemesis in Williams.

Sharapova, who celebrates her 21st birthday on Saturday, has now lost two of her seven encounters with the former world number one.

Her last win came in 2004.