LONDON (AFP) - World No. 1 Novak Djokovic apologised for upsetting a ballgirl who looked visibly shaken when he roared at her during his Wimbledon fourth round win over Kevin Anderson on Tuesday.

The defending champion was 3-2 down and at 30-30 in the final set when he screamed in the girl's direction.

"I'm sorry. There was nothing towards her. Maybe she was just afraid of my screaming there," said Djokovic who said he was trying to fire himself up in the fifth set decider.

"I was pretty close to her. I'm definitely going to try to apologise to her if I did something wrong."

Earlier this year, the Serbian star had to apologise to a ballboy in Miami after the youngster got caught in the crossfire of a vociferous exchange between the player and his coaching team in the stands during his final win against Andy Murray.

Djokovic beat Anderson 6-7 (6/8), 6-7 (6/8), 6-1, 6-4, 7-5 to set-up a quarter-final clash against Croatia's Marin Cilic on Wednesday.