Serena Williams pulled out of her Fed Cup match against Argentina on Sunday, citing an illness.
Williams, who won the Australian Open singles title last weekend, was set to face Paula Ormaechea in Buenos Aires but was replaced in the U.S. lineup by Coco Vandeweghe.
In the first round of singles on Saturday, Serena defeated Maria Irigoyen 7-5, 6-0, while sister Venus downed Ormaechea 6-3, 6-2.
The U.S. needs one win in Sunday's three matches — two singles, one doubles — to advance.
The U.S. is trying to get back into the top-tier World Group in the Fed Cup after dropping out with an upset loss to France last year. The Americans have won a record 17 Fed Cup titles, but the last one came in 2000.
