MELBOURNE, Australia -- Last year, Angelique Kerber became the oldest first-time No. 1 in women's tennis, and Wednesday she celebrated her 29th birthday with a nervous 6-2, 6-7 (3), 6-2 victory over Carina Witthoeft.

It was the second consecutive three-set affair for Kerber, who was also pushed to the limit by Lesia Tsurenko in the first round.

"To have this pressure, it's a privilege," Kerber, a native of Germany, said afterward. "It's new for me, but I think I am dealing with it pretty well."

This was Kerber's 16th straight win at a hard-court major, going back to her triumphs at the 2016 Australian Open and US Open.

Afterward, the crowd at Rod Laver Arena happily serenaded Kerber with "Happy Birthday."

"Actually, I'm always playing on my birthday," Kerber said. "I'm getting older, but yeah, I think I will have a great day."

Kerber managed to avoid an upset of historic proportions; Witthoeft would have been the fourth-lowest-ranked player in the Open era to defeat a world No. 1 at a major.

The match ran 2 hours, 8 minutes on a day that was some 30 degrees cooler than Tuesday. Kerber broke Witthoeft's serve on seven of 14 opportunities.

"I was doing, I think, a lot of mistakes like in the important moments," Kerber said. "But at the end, I'm happy that I won the match. It was not so easy. The [breezy] conditions were, like, also a little bit difficult today."

Next she'll play Kristyna Pliskova in Friday's third round, but she said she was not concerned about already playing two three-set matches.

"I think I'm fit," Kerber said. "I am prepared for playing, like, two, three hours, so that's not a problem. Tomorrow I have a day off, so it's not a big deal."

Karolina Pliskova -- Kristyna's twin sister -- won her second-round match, 6-0, 6-2, over Anna Blinkova last just 59 minutes. She will next play Jelena Ostapenko, who beat No. 31 Yulia Putintseva 6-3, 6-1.

Pliskova overwhelmed the 18-year-old Blinkova, a Russian qualifier ranked 189th who had beaten veteran Monica Niculescu in the first round.

When Blinkova won her first game -- after Pliskova had won 10 in a row -- she smiled and raised her left arm to acknowledge the cheers from the crowd. The fifth-seeded Pliskova advanced to the third round for the third consecutive year.

"It's always good to be in the zone,'' said Pliskova, who won the Brisbane International title earlier this month. "It can always be a bit better.''

No. 11 Elina Svitolina had a 6-4, 6-1 win over U.S. qualifier Julia Boserup to advance to a third-round match against No. 24 Anastasia Pavlyuchenkova, a 6-2, 6-2 winner over fellow Russian Natalia Vikhlyantseva.

World top-ranked Angelique Kerber prevailed in a tight three-set match against fellow German Carina Witthoeft at Rod Laver Arena. PAUL CROCK/AFP/Getty Images

In other matches, Alison Riske beat No. 20 Zhang Shuai 7-6 (7), 4-6, 6-1, and Mona Barthel beat Olympic gold medalist Monica Puig.

Sorana Cirstea defeated Carla Suarez Navarro 7-6 (1), 6-3. Australian wild-card entry Ash Barty defeated American Shelby Rogers 7-5, 6-1 to reach the third round for the first time.

Eugenie Bouchard is back in the third round at Melbourne Park for the first time in two years, defeating China's Peng Shuai 7-6 (5), 6-2.

Bouchard, who reached the semifinals of the Australian Open in 2014, finally closed it out after Peng saved three match points on Bouchard's serve at 5-1 in the second set.

Following her breakout year in 2014, which also saw her reach the Wimbledon final, Bouchard struggled to make it past the fourth round at the slams. Her best result last year was the third round at Wimbledon. Her next opponent will be CoCo Vandeweghe, who defeated Pauline Parmentier, 6-4, 7-5.

"Overall, I'm feeling better with each passing day," Bouchard said.

French Open champion Garbine Muguruza advanced to the third round with a 7-5, 6-4 win over American Samantha Crawford. Muguruza clinched the match on her first match point when Crawford netted a forehand. The Spaniard will play Anastasija Sevastova in the third round.

WTA Finals winner and no. 6 seed Dominika Cibulkova held off Hsieh Su-wei 6-4, 7-6 (8) and will next play No. 30 Ekaterina Makarova, who was leading 6-2, 3-2 when Sara Errani retired because of a leg injury.

Two-time major winner Svetlana Kuznetsova will meet former No. 1-ranked Jelena Jankovic.

Jennifer Brady beat Heather Watson 2-6, 7-6 (3), 10-8 to advance to the third round of a Grand Slam for the first time. Brady will take 14th-seeded Elena Vesnina of Russia.

The Associated Press contributed to this report.