John Isner withdrew from the Hopman Cup on Thursday with a right knee injury, ending the American team's chances of reaching the final of the eight-country tournament and handing a championship berth to Spain.
Isner, who still hopes to compete at the Australian Open beginning Jan. 14, played with strapping 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.
Isner said after withdrawing on Thursday, however, that he's not sure if tendinitis is the problem.
The American, who still hopes to compete at the Australian Open, played with strapping on his knee when he lost to Frenchman Tsonga.
"I don't necessarily know if it's tendinitis," he said. "It's sore and it's caused me a little bit of trouble and from that I'm not able to really load the right way and I'm not really going for my shots like I should."
Isner, the highest-ranked American at No. 14 in the world, 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. 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 (at the Australian Open)," he added.
Spain and the US were each 2-0 in round-robin play at the Hopman Cup heading into Thursday's deciding match-up. Spain was awarded a walkover and will now advance to Saturday's final against an as-yet undetermined team from Group A.
Although Spain is already assured of advancing, Isner was replaced by his Australian hitting partner, Thanasi Kokkinakis, for his singles match against Fernando Verdasco.
Venus Williams was also still scheduled to play Spain's Anabel Medina Garrigues in women's singles and would team with Kokkinakis to play Verdasco and Medina Garrigues in a meaningless mixed doubles match.