Novak Djokovic was hit on the head by a water bottle as he signed autographs after his win at the Italian Open.
A video on social media showed world number one Djokovic falling to the floor after being hit by the object, which came from the stands.
The 24-time Grand Slam champion stayed on the ground for a while before being helped out of the arena in Rome by security.
Tournament organisers said the hard bottle had fallen out of the bag of a boy in the crowd.
"This is not being treated as a deliberate act, but as an accident," said a statement reported by PA.