WASHINGTON _ Serena Williams is pulling out of the United States' Fed Cup semifinal against Russia this weekend because she injured herself during Wimbledon, The Associated Press learned on Wednesday.

The eight-time Grand Slam singles champion will be replaced by Meilen Tu, who flew to Stowe, 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 Thursday. The draw for the best-of-five semifinal is on Friday, with matches on the weekend.

The matchup already lost some luster when two-time major champion Maria Sharapova, who is No. 2 in the rankings, withdrew from Russia's team 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, Russia 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.

Williams and her older sister Venus were part of the four-player team announced last month by U.S. captain Zina Garrison, along with doubles specialist Lisa Raymond and teenager Vania King. But five days later, Serena Williams strained her left calf and hurt her left wrist and thumb during her fourth-round victory at Wimbledon.

Particularly hampered by the wrist and thumb _ she had to hit backhands with one hand instead of two _ she lost to Justine Henin in the quarterfinals at the All England Club. On Tuesday, Williams pulled out of next week's WTA tournament in Mason, Ohio.

She and Venus, who won Wimbledon for her sixth major title, were on the U.S. team that beat Belgium 5-0 in the Fed Cup quarterfinals at Delray Beach, Florida in April. That was Serena Williams' first Fed Cup action since 2003, and Garrison has spoken about the sisters' commitment to participate in the competition and win it _ if healthy.

The United States hasn't won the Fed Cup title since 2000.

The 41st-ranked Tu complained last month that she hadn't heard at all from Garrison before the team was picked.