MELBOURNE, Australia -- Top-ranked Simona Halep came back after dropping the second set and going down a break in the third to beat Sofia Kenin 6-3, 6-7 (5), 6-4 and move into a third-round match against Venus Williams.
Halep reached the final in Australia last year after saving match points in the third round and again in the semifinals, but lost the championship match to Caroline Wozniacki.
She ended a run of three runner-up finishes at the majors by winning the French Open in her next trip to a Grand Slam event.
Kenin went on a roll in the third set and was leading 4-2, but gave Halep a break point chance with a double-fault and allowed the Romanian to get back on serve when she hit a wayward forehand.
Halep broke for a 5-4 lead -- after Kenin saved two break point chances -- and calmly closed it out in 2 hours, 31 minutes.
"I have no idea how I won tonight," Halep said. "So hard to explain what happened on court."
Seven-time major winner Venus Williams advanced with a 6-3, 4-6, 6-0 win over Alize Cornet. Serena Williams looms as a fourth-round opponent for the winner of Venus versus Halep.
Williams needed only 70 minutes to beat 2014 Wimbledon finalist Eugenie Bouchard 6-2, 6-2.
Williams is in Australia aiming for a 24th Grand Slam singles title. She won here in 2017 but missed her title defense last year while she took time off following the birth of her daughter.
"It wasn't an easy match tonight -- Genie has been to the finals at Wimbledon," Williams said.
Williams, 37, will next play Dayana Yastremska, who beat No. 23 Carla Suarez Navarro 6-3, 3-6, 6-1.
Also on to the third round is US Open champion Naomi Osaka, after beating Tamara Zidansek 6-2, 6-4.
The fourth-seeded Osaka recovered a service break in the second set and, at 4-4, she broke Zidansek's service to love. Osaka hit an ace to save a break point in the last game and clinched it on her second match point with a service winner.
Osaka began the year with a semifinal appearance at the Brisbane International, losing to eventual finalist Lesia Tsurenko.
She has reached the semifinals at four of her past five tournaments. She will next play No. 28-seeded Su-Wei Hsieh, who beat Laura Siegemund 6-3, 6-4.
Earlier, 2016 US Open finalist Karolina Pliskova extended her winning start to the season after a rain-interrupted 4-6, 6-1, 6-0 win over Madison Brengle.
Pliskova started this year with a run to the title at the Brisbane International and entered the season's first major seeded seventh, still with a chance to regain the top ranking, depending on her performance at Melbourne Park.
After dropping serve twice in the first set against Brengle, Pliskova dominated the second and third sets, and capped it with a pair of aces.
"Some breaks, some good tennis and some bad mistakes from me, too,'' Pliskova said. "The first set wasn't very good from my side, but she played some good points and was very solid.''
She'll next play No. 27 Camila Giorgi, who beat Iga Swiatek 6-2, 6-0.
Madison Keys, a 2017 US Open finalist, had a 6-3, 6-4 win over Anastasia Potapova to set up a third-round match against No. 12-seeded Elise Mertens, who beat Margarita Gasparyan 6-1, 7-5.
Keys has reached the semifinals in three of the past five majors.
In other early matches, WTA Finals winner Elina Svitolina beat Viktoria Kuzmova 6-4, 6-1 in 67 minutes; No. 13 Anastasija Sevastova beat teenage Canadian qualifier Bianca Andreescu 6-3, 3-6, 6-2; and No. 21 Qiang Wang had a 6-2, 6-3 win over Aleksandra Krunic.
The Associated Press contributed to this report.