Novak Djokovic wasn't optimistic about his right ankle injury after lifting Serbia to an insurmountable 3-1 lead over the United States with a 7-5 6-7 (7-4) 6-1 6-0 win over Sam Querrey in a Davis Cup quarter-final.

The world No.1 twisted the ankle while hitting a forehand in the third game of the match on Sunday.

"Injuries are part of the sport and you try to avoid them as an athlete as much as you can," Djokovic said. "The nature of the injury is still to be determined.

"One hour, one hour and a half after the end of the match, all I can say is it doesn't look good."

Djokovic had the ankle tightly wrapped and had taken anti-inflammatory pills. He plans to travel home to Monte Carlo to have an MRI scan.