Perth - A knee injury forced world number 14 John Isner to withdraw from the Hopman Cup on Thursday, throwing his Australian Open preparations into doubt.
The early season setback for the towering American number one came 11 days ahead of the start of the year's first Grand Slam on January 14 in Melbourne.
Isner, six feet nine inches (2.06 metres) tall, looked restricted in both his singles matches for the United States in the mixed teams event, losing in straight sets to South Africa's Kevin Anderson and Jo-Wilfried Tsonga of France.
After his loss to Tsonga on Tuesday, Isner conceded his right knee was sore, but said it was not affecting his play and was not a major concern.
"My knee's been bothering me a little bit but it's fine," he said. "It's a little tendonitis or something."
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.