World number one Serena Williams has admitted the she lacked pace and intensity on the court following her Madrid Open semi-final exit to Petra Kvitova.

The American suffered her first defeat of the year in a 6-2 6-3 loss at the hands of the Czech, who recently returned from a six-week break.

"I think she just played well today," Williams told reporters. "She went for broke on every serve, every return. I hit some second serves, 170, 175, and she just hit them for winners.

"I was very slow off the mark. After my serve I wasn't moving as well as I should have. You know, I wasn't in it. I wasn't Serena today. I think that was the main thing."

Earlier in the day, Maria Sharapova was knocked out by Svetlana Kuznetsova.