NEW HAVEN, Conn. -- Kim Clijsters dropped out of the Pilot Pen tournament on Friday with a right thigh strain.

Clijsters, seeded fourth, said she wants to get as much rest and medical treatment as she can before her match against Pavlina Nola of Bulgaria in the U.S. Open.

"I feel really bad it has to end like this," said Clijsters, who apologized to the fans at the Connecticut Tennis Center before retiring.

Clijsters, an 18-year-old Belgian, said she felt a pain "like someone hit an elbow into your leg" Thursday when chasing a wide serve to her backhand in a quarterfinal match with Nathalie Tauziat.

After the changeover, she said she felt a sharp pain, as if her leg would cramp. Clijsters beat her French opponent 6-1, 7-6 (3).

She got ice massages for the leg Thursday night. When she woke up Friday, the leg was very tight. Warming up in practice made it worse, she said.

"At the end of practice I sort of started feeling it with every step I was taking," Clijsters said.

Clijsters planned to get treatment with the physical therapists at the U.S. Open in New York later Friday.

Her semifinal opponent, top-seeded Lindsay Davenport, advanced to the final Saturday. She will play either second-seeded Jennifer Capriati or third-seeded Venus Williams, who will meet in semifinal Friday evening.

Williams defeated Justine Henin of Belgium 6-3, 5-7, 6-2 in the quarterfinals Friday afternoon.