World number 14 John Isner expects to be fit to play at January's Australian Open, despite being forced out of the Hopman Cup by a knee injury.
The towering American withdrew from the mixed team event on Thursday, having looked restricted in both his singles matches for the United States, losing in straight sets to South Africa's Kevin Anderson and Jo-Wilfried Tsonga of France.
After his defeat to Tsonga in Perth on Tuesday, the 27-year-old American number one conceded his right knee was sore but said it was not affecting his play.
However, Isner, six feet nine inches (2.06 metres) tall, conceded on Thursday that his knee was hampering him.
"Well I don't actually know if it's tendonitis, it's just sore," he said.
"It's causing me a little bit of trouble and from that I'm not able to, to really load the right way and you know I'm not really going for my shots like I should."
The Australian Open, the year's first Grand Slam, starts on January 14 and Isner said he expected to be fit to play in Melbourne, despite not being certain of the nature of the injury.
"(I will) treat it the best I can and see how it reacts," he said.
"I don't know exactly. I wish I did know but I do know that I've always been a fast healer when I have had real injuries pop up here and there so I'm confident I'm going to be fine for the Australian Open."
His withdrawal also ends Venus Williams's hopes of joining her sister, Serena, as a Hopman Cup winner, with the United States forfeiting their Group B tie against Spain on Thursday, handing the Spaniards a berth in Saturday's final.