Angelique Kerber foiled the threat of gate-crashing compatriot Carina Witthoeft before celebrating her 29th birthday and keeping her Australian Open title defence alive.

The faltering world No.1 conceded the last six points of the second-set tiebreak and the first five points of the third set before steadying to win 6-2 6-7 (3-7) 6-2 on Wednesday and reach the third round.

It was the 10th successive occasion Kerber had beaten a German opponent, but this time she did it the hard way.

“I’m 29 and getting older but I think I will have a great day,” said Kerber, who was serenaded with a rousing chorus of Happy Birthday by the Rod Laver Arena crowd.

Witthoeft made a dreadful start to the match, double-faulting six times in her opening two service games.

But after scorching through the second-set tiebreak and going on a run of 11 straight points the free-swinging 21-year-old threatened to cause a huge boilover.

“It’s a completely new situation for me (being the defending champion) but I think that I’m dealing with it very well and enjoying it,” said Kerber, who will play Czech Kristyna Pliskova on Friday in the round of 32.

“I was trying to play more aggressively, but today I was doing a few more mistakes.”

Due to the scheduling of the first major of the year, Kerber has spent her past 10 birthdays in Melbourne.

“I remember that I celebrated my 18th birthday at home, so that was a big party,” she said.

“But the rest, I was always here.

“But I feel at home – it’s great to be always in Australia for my birthday.”