The world number one went over on his ankle in the third game of Sunday's rubber against Sam Querrey. He had to be helped to his chair and then received lengthy treatment.
Djokovic was able to continue and despite seeming troubled by the problem he went on to post a 7-5 6-7 (4/7) 6-1 6-0 victory, one which booked Serbia's place in the semi-finals.
Quizzed about the injury afterwards, Djokovic told the Associated Press: "One hour, one hour and a half after the end of the match, all I can say is it doesn't look good.
"It's swollen, and when it's swollen it's definitely not a good first sign."
Djokovic said he would try his hardest to be fit in time to play the Monte Carlo Masters, the first big event of the claycourt season, which gets under way on Sunday.
He added: "I'm going to do everything I can in my power to recover till that tournament. How realistic that is, I don't know what to tell you."