Serena Williams pulled out of the United States' Fed Cup semifinal against Russia at Stowe, Vt., this weekend because she injured herself during Wimbledon, The Associated Press learned Wednesday.

The eight-time Grand Slam singles champion will be replaced on the U.S. roster by Meilen Tu, who flew to Vermont on Wednesday to join the rest of the squad, a person at the U.S. Tennis Association told the AP.

The person spoke on condition of anonymity because Williams' withdrawal and replacement by Tu were not going to be announced formally until today. The draw for the best-of-five semifinal is Friday, with matches Saturday and Sunday.

The matchup already lost some luster when two-time major champion Maria Sharapova, who is No. 2 in the rankings, withdrew from Russia's roster because of a right shoulder injury that's bothered her for months. Russian media reported that 2004 U.S. Open champion Svetlana Kuznetsova would pull out, too.

There was, however, some lingering intrigue: As of Wednesday, Russian captain Shamil Tarpishchev was still waiting to get a visa to be able to travel to the United States, and Russia's Foreign Ministry posted a statement on its Web site saying, "We consider this slowness to be incomprehensible and unwarranted."

The USTA said it would help Tarpishchev in any way it could.