(CNN)Serena Williams suffered her first defeat of 2015 after going down 6-2 6-3 to Petra Kvitova in the semifinals of the Madrid Open on Friday.
Serena Williams and Maria Sharapova defeated in Madrid Open semis
The 19- time grand slam champion had won a remarkable 27 matches on the bounce -- a run stretching back to October 2014 -- but struggled all day against her powerful opponent on the Madrid clay.
It was also her first loss to Kvitova in five previous encounters.
"I think she (Kvitova) just played well today," Williams later told reporters when asked to assess the match. "She went for broke on every serve, every return. I hit some second serves, 170, 175, and she just hit them for winners."
But while magnanimous towards her opponent, Williams was also critical of her own display. "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."
Williams shock departure overshadowed another surprise defeat in the day's other semifinal, that of reigning Madrid champion Maria Sharopova.
The World No. 3 was swept aside 6-2 6-4 by Svetlana Kuznetsova.
With the French Open beginning at the end of the month, both Williams and Sharapova will be looking to sharpen up ahead of the year's second grand slam.
Sharapova, who will be defending her title at Roland Garros, has struggled at the beginning of the 2015 clay court season.
The five-time grand slam winner lost in her first outing at the Stuttgart Open at the end of last month but remained positive about her preparation for the French.
"Obviously we have another tournament ahead of us next week (in Rome) which will also be an important part of the preparation," Sharapova told reporters.
"I think every match for any player at this stage before a Grand Slam is important, so it will be for me as well," she added.