Novak Djokovic has promised to apologise to a Wimbledon ball girl after angrily yelling towards her during his fourth-round win over Kevin Anderson.

The defending champion appeared to shout “towel” aggressively at the startled youngster when struggling to fend off the big-serving South African.

Djokovic won through 6-7, 6-7, 6-1, 6-4, 7-5, resuming on Tuesday at two sets all after fading light had brought the match on No1 Court to an overnight halt.

When asked about the incident involving the ball girl, which came in the sixth game of the deciding set, after he planted a loose forehand into the net to end a 20-shot rally, Djokovic said: “I’m sorry. There was nothing towards her.

“Maybe she was just afraid of my screaming there. I was pretty close to her. I’m definitely going to try to apologise to her if I did something wrong.”

Djokovic’s safe progress to the quarter-finals came on the 30th anniversary of the coach Boris Becker’s first Wimbledon triumph as a 17-year-old.

“I didn’t know it was today. But he’s going to have a glass of wine, I’ll have a glass of water, and we will celebrate this nice milestone,” Djokovic said.

Becker remarked of Djokovic’s performance: “I think that’s a message to the locker room that whoever said he wasn’t ready before, well he’s ready now.”

The 28-year-old world No1 will face the US Open champion Marin Cilic on Wednesday in the quarter-finals. They have played 12 times previously, and Djokovic has prevailed each time.

The Serb will be looking to avoid an unlucky 13th, and he said of his Croatian rival: “He had an exceptional season last year, the best of his life. He played very well at the grand slams, and of course won the US Open. That’s the crown of his career.

“I remember him mentioning earlier this year that he wants to focus mostly on Wimbledon, grass. That’s where he feels like he has a best chance besides the US Open.”