Australian Open 2017: Angelique Kerber and Venus Williams reach third round

Witthoeft (right) had lost both her previous meetings with Kerber

Defending champion Angelique Kerber survived an onslaught from fellow German Carina Witthoeft to advance to the Australian Open third round.

Kerber - top seed at a Grand Slam for the first time - struggled with her serve in a second-set tie-break before prevailing 6-2 6-7 (3-7) 6-2.

The top seed, who was 29 on Wednesday, faces Czech Kristyna Pliskova next.

Venus Williams beat Swiss qualifier Stefanie Voegele 6-3 6-2 to reach the third round for the 13th time.

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Venus still going strong at 36

Williams, who played at her first Australian Open in 1998, is the oldest woman in the singles draw at 36 and is competing in her 73rd Grand Slam.

"I have to talk about my age every interview!" the American said. "I've played some of the greats.

"It's an honour and privilege to start that young, and play this old."

She later pulled out of the doubles competition with her sister Serena as a precaution to rest a sore elbow.

The 17th seed has never won the title in Melbourne, her best result finishing as runner-up to Serena in 2003.

She will next play Duan Yingying after the Chinese player beat Varvara Lepchenko 6-1 3-6 10-8.

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'Pressure is a privilege'

Kerber was presented with a cake to celebrate her birthday

World number one Kerber has started the year in less-than-convincing style, going out in the last eight in Brisbane and the second round in Sydney.

And Witthoeft, 21, posed a far more serious challenge in this meeting than in the 6-0 6-0 defeat she suffered against her compatriot at Wimbledon in 2015.

After a frustrated Kerber coughed up successive double faults in the tie-break, Witthoeft's powerful groundstrokes took the opening game of the decider against serve.

But Kerber won the next four to regain control and avoid an upset.

"To have this pressure is a privilege," she said. "It's completely new for me, but I'm doing well. I'm just trying to enjoy it."

Best of the rest

World number seven Garbine Muguruza advanced to the third round by beating America's Samantha Crawford 7-5 6-4.

Meanwhile, Russia's Svetlana Kuznetsova, a three-time quarter-finalist in Melbourne, was a 6-2 6-1 victor over Australia's Jaimee Fourlis.

But 10th seed Carla Suarez Navarro is out after a shock defeat by Romania's world number 78 Sorana Cirstea.

The Spaniard went down 7-6 (7-1) 6-3.

Canadian Eugenie Bouchard avoided a second successive Australian Open second-round exit by beating China's Peng Shuai.

Bouchard, a Melbourne semi-finalist in 2014 who is now ranked 47 in the world, won 7-6 (7-5) 6-2.

Australia's Ashleigh Barty, in her first Australian Open appearance since 2014, moved to the third round for the first time with a 7-5 6-1 win over American Shelby Rogers.