Isner, who still hopes to compete at the Australian Open beginning Jan. 14, played with tape on his knee when he lost to Jo-Wilfried Tsonga of France on Tuesday and later said he was suffering from “a little tendinitis.”
He also lost to South African Kevin Anderson in his opening match on Sunday.
The highest-ranked American at No. 14, Isner said he still plans to play in next week’s Sydney International and the Australian Open.
“My plan now is just to treat it as best I can and see how it reacts,” Isner said. “I do know I’ve always been a fast healer when I’ve had little injuries pop up here and there, so I’m confident I’m going to be fine for the Australian Open.
“It’s not a great way to start the year but I’d rather it happen this week than in two weeks.”
Later Thursday, Bernard Tomic followed up his victory over No. 1 Novak Djokovic by beating Andreas Seppi 6-3, 7-5 and Ashleigh Barty defeated Francesca Schiavone 6-0, 6-3 to help Australia beat Italy.
Australia and Serbia, which plays Germany today, are bidding to meet Spain in Saturday’s final.
Spain and the United States were 2-0 in round-robin play heading into Thursday’s match, but Spain was awarded a walkover after Isner’s injury.
Isner was replaced by his Australian hitting partner, Thanasi Kokkinakis, for an exhibition singles match against Fernando Verdasco. The Spaniard won 7-6 (4), 6-3.
Venus Williams also remained undefeated in her three singles matches as she recovered from a slow start to beat Spain’s Anabel Medina Garrigues 6-3, 6-4.
After dropping the opening game on a double-fault, Williams was treated for a blister on her left foot and also taped her right wrist. She then broke to level the set at 2-2 and again at love to lead 5-3.
Williams broke the Spaniard’s serve two more times in the second set to earn a comfortable victory.
“I got to play a lot and pull a lot out of myself and I feel that with every match my serve got better,” Williams said. “This is the best day I’ve returned, my consistency (is good),” Williams said. “That’s what I wanted.”
She said her blisters were not a problem after they were treated.
“Once I got that taken care of I was able to run a lot better,” she said. “And once I got my wrist wrapped, that was fine, too.”