Big-serving John Isner withdrew from the Hopman Cup on Thursday with a knee injury that dashed the Americans' hopes of reaching the final of the mixed team event in Perth.

Isner was battling what he called "a bit of tendonitis" all week and his withdrawal ahead of the singles match against Spain's Fernando Verdasco also ended teammate Venus Williams' hopes of emulating sister Serena's 2008 Hopman Cup victory.

"I'm very very sorry to all the fans here, so I apologize for that and I hope you guys understand," said Isner, who became the second player to quit the tournament.

His withdrawal handed Spain a 3-0 victory and passage into the final for the sixth time. Spain have won the mixed teams title three times, most recently in 2010, when Tommy Robredo and Maria Jose Martinez Sanchez lifted the title against Great Britain's Laura Robson and Andy Murray.

"I absolutely love this event, it's my third time here and it's very disappointing for me and it's a tough one to swallow but for me, first and foremost, I've got to try and get 100 percent healthy," Isner said.

Isner was optimistic of playing the Australian Open later this month.

"My plan now is just to treat it as best I can and see how it reacts," he 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 (at the Australian Open)."

With Isner out, Verdasco found himself playing against Isner's little-known Australian hitting partner, 16-year-old Thanasi Kokkinakis, winning in straight sets despite an unexpectedly spirited showing from the youngster.

Reuters - Global Times