Djokovic offers to apologise to upset Wimbledon ballgirl

Wimbledon champion Novak Djokovic promised to apologise to a shaken ball girl for an outburst during his tough five set victory over big-serving South African Kevin Anderson on Tuesday.

The world number one, who had fought back from two sets down on Monday in a match halted by bad light, let rip midway through the tense final set.

Asked about the incident, he expressed surprise and said the anger was aimed at himself, although he appeared to be calling for a towel and gesticulating angrily at the ballgirl.

"I'm sorry. There was nothing towards her," the top seed said after reaching a 25th successive Grand Slam quarter-final with what he called one of the toughest matches of his Wimbledon career.

"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 had to do the same to a ball boy after a similar incident during this year's Miami Open final when he raged in the direction of his entourage in the players' box and snatched away a towel.

Sometimes, the Serb told reporters, he could not hold it all in.

"I just took out everything I had, not on anybody but me. I was looking at the box, but I was talking to myself," he said of Tuesday's incident.

"I was trying to get myself motivated. I was not showing much emotion yesterday. I was just trying to keep it together... sometimes it's just good to scream and let it all out, because that's the way I work."

Anderson's ballistic serve was something else to scream about, the South African blasting 40 aces and serving at speeds of up to 139mph in a match lasting three hours and 47 minutes.

"It's particularly frustrating when you're playing somebody that serves this well and doesn't give you any look at the break," said Djokovic.

He explained also that he had not slept well, leaving the court on Monday buzzing with adrenalin and still awake past midnight.

"Obviously, the fifth set today was very frustrating, very tense. I didn't have basically any chances on his serve until that 11th game when he made two double-faults and I made a couple of good returns from the forehand side," he said.

"Until the last moment, until the last point, I didn't know if I was going to win or not."