NEW YORK -- Serena Williams withdrew from the U.S. team's first-round Fed Cup match at Slovenia.
The U.S. Tennis Association on Wednesday couldn't say why she pulled out, and a call to her publicist was not immediately returned.
Williams had been named to the squad by captain Zina Garrison in March, but she was not on the International Tennis Federation's official team nomination list Wednesday.
Laura Granville was listed as her replacement for the April 24-25 series. Venus Williams, Lisa Raymond and 47-year-old Martina Navratilova also are on the team.
Williams is competing at the Family Circle Cup in Charleston, S.C., but she wasn't playing Wednesday and not able to be reached for comment.
Williams returned to tennis last month after an eight-month injury absence. She had left knee surgery in August. Williams has won six Grand Slam titles, including four in a row from 2002-03.
On Tuesday, Williams defeated Kelly McCain 6-1, 6-0 in the second round in Charleston.
Top-ranked Justine Henin-Hardenne also will miss the first round of the Fed Cup. The Belgian, who won at Indian Wells, Calif., last month, withdrew from the Family Circle tournament Tuesday because of dizzy spells from a low blood-sugar count.
Belgium, the 2001 winner, will now rely on second-ranked Kim Clijsters. She will be joined by Els Callens, Kirsten Flipkens and Elke Clijsters against Croatia's Karolina Sprem, Jelena Kostanic, Silvija Talaja and Iva Majoli.
Third-ranked Amelie Mauresmo will lead defending champion France against Germany on clay in Amiens, France, on April 24-25. Mauresmo will team with Nathalie Dechy, Mary Pierce and Emilie Loit, while Germany counters with Marlene Weingartner, Anna-Lena Groenefeld, Julia Schruff and Barbara Rittner.
In other matchups, it's Australia at Russia, Switzerland at Spain, Slovakia at Austria, Japan at Argentina and the Czech Republic at Italy.