Rafael Nadal celebrates defeating Tomas Berdych in the Rome quarterfinals.
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ROME (AP) -- A week after Novak Djokovic and Rafael Nadal lost in the Madrid quarterfinals on an experimental blue court, both showed they remain a force on red clay Friday by reaching the Italian Open semifinals.
The top-ranked Djokovic struggled with his serve in the opening set but found his range to eliminate fifth-seeded Jo-Wilfried Tsonga 7-5, 6-1. Nadal defeated seventh-seeded Tomas Berdych 6-4, 7-5.
Djokovic, the defending champion, will next play the winner of the match between Roger Federer and Italian hope Andreas Seppi. Nadal's semifinal opponent is sixth-seeded David Ferrer, who beat Richard Gasquet 7-6 (4), 6-3.
Among the women, Serena Williams reached the last four after Flavia Pennetta quit with an injured right wrist while trailing 4-0 in the first set. Williams' winning streak is up to 17 matches following titles in Charleston, S.C., and Madrid. She also has two victories in Fed Cup. Her next opponent is French Open champion Li Na, who eliminated No. 14 Dominika Cibulkova 6-1, 7-6 (4).
Defending champion Maria Sharapova beat Venus Williams 6-4, 6-3. Wimbledon champion Petra Kvitova will play No. 12 Angelique Kerber later Friday.
While Djokovic has not been dominating as he did last year, the Serb still finds a way to win without playing his best. Serving at 5-5 in the first set, he saved a potentially decisive break point with an authoritative serve and volley. Then he took six straight points to win.
Down to No. 3 in the rankings this week, Nadal broke in the opening game to take control of the first set. He recovered a break midway through the second set before pulling ahead for good. Berdych was coming off a runner-up performance at the Madrid Open. He lost to Federer, who then replaced Nadal at No. 2.
Pennetta called for the trainer after falling behind 3-0. Her wrist was retaped but the Italian conceded during the fifth game after 28 minutes of play.
"I felt pain during the second or third point of the opening game,'' Pennetta said. "Something happened on the backhand. ... Then not being able to serve and attack like usual, there wasn't much else to do.''
For Serena, who won this tournament 10 years ago, the quick match was welcome.
"I have played every day and I haven't had a day off,'' she said.