Serena Williams has withdrawn from US Fed Cup team for next week's final against Italy in Calabria, the USTA said on Saturday.
No reason for her withdrawal was given in the statement which said Vania King woulld take her place.
Serena, who will reclaim the top ranking on Monday, has not played for the US in the Fed Cup since 2007.
The US are trying to win the competition for the first time since 2000 but Serena's withdrawal will severely weaken their chances against a strong Italy side.
King will join teenager Melanie Oudin, a quarter-finalist at the U.S. Open last month, doubles specialist Liezel Huber and Alexa Glatch.
Williams will take on Caroline Wozniacki later on Saturday in the semi-finals of the WTA Championships.