Djokovic coasted through the first two sets, winning 7-6 and 6-4.
But after a close-fought start to the third, he appeared to be struggling with an issue in his chest.
The world number one appeared unable to move for any shots on the forehand side, and struggled to move about the court.
He managed to maintain his composure and keep hitting shots when he could, but it was clear to see he was grimacing whenever he had to move to the right.
Djokovic then had a medical timeout and appeared to have some treatment to his right lower rib.
His serve speed dropped dramatically – by 11mph – after he suffered the injury.
His trainer came onto the court to help him and rub some sort of ointment onto his skin, but Djokovic still didn’t look comfortable when he returned to play.
On his service game, Djokovic made numerous unforced errors and looked to be uncomfortable even standing and hitting forehand shots.