Reigning champion Madison Keys overcame a second-set wobble against Ashlyn Krueger to advance to the third round of the Australian Open.
Keys, the ninth seed in the draw, notched up a 6-1 7-5 win over fellow American Krueger but was less than convincing after powering her way through the opening set.
The defending champion allowed doubts to creep into her game in the second set, handing Krueger two breaks on serve with a succession of double faults before battling back to progress to the next round to face Karolina Pliskova.
"I started really well, and I think Ashlyn started a little slow, and then I was fully expecting her to raise her level, which she did," Keys said.
"Once I kind of got that momentum, I tried to sink my teeth into the set, and do whatever I could to get back into it."
Iga Swiatek (above) resumed her quest for a first Australian Open title and career grand slam with a 6-2, 6-3 victory over unseeded Marie Bouzkova, as she showed no signs of the sluggishness that dogged her in the last round.
The 24-year-old arrived in Melbourne after two losses during Poland's triumphant United Cup run and had to fight past Chinese qualifier Yuan Yue in the last round, but normal service looked to have resumed against Bouzkova in John Cain Arena.
Swiatek eased through the first set, during which she raised the intensity early on and made several successful forays to the net, but the six-times major champion found herself on the back foot in the second set as Bouzkova surged to a 3-1 lead.
The second seed got back on level terms after six games and then nailed a forehand winner at the net to edge ahead, going on to seal the win from there and setting up a third-round clash with 31st seed Anna Kalinskaya.
Naomi Osaka dropped a set but overcame Romania's Sorana Cirstea 6-3 4-6 6-2.
Osaka, who won the Australian Open in 2019 and 2021, made a shaky start against Cirstea, dropping the opening game on her serve and committing early errors.
She soon shook off the cobwebs and broke to level at 2-2, before breaking again for a 5-3 lead and then defending a break point to hold serve and take the opening set.
The 35-year-old Cirstea, who will retire at the end of the year, again took an early lead in the second set and though Osaka levelled at 2-2, the Romanian broke in a decisive moment to force a final set.
Osaka, the 16th seed, took a break between sets and then another medical timeout while leading 3-1 in the decider, but any concern was short-lived.
With her movement seemingly hampered, the Japanese former world number one responded by dialling up the intensity of her ground strokes and overwhelming Cirstea to seal the win.
The final set was also tinged with some drama as Cirstea took issue with Osaka pumping herself up by shouting "come on" between the Romanian's first and second serves.
"Apparently a lot of 'come ons' that she was angry about. I mean I tried to play well. I think I hit a lot of unforced errors but I tried my best," Osaka said.
"She's a great player. I think this was her last Australian Open, so sorry she was mad about it."
Sixth seed Jessica Pegula had few issues against doubles partner McCartney Kessler, handing out a comprehensive 6-0 6-2 defeat to her fellow American in 58 minutes and set up a meeting with Russia's Oksana Selekhmeteva in the third round.
Amanda Anisimova, also from the United States, booked her place in the next phase with a 6-1 6-4 win over Katerina Siniakova and the fourth seed will next face compatriot Peyton Stearns.
Kazakh fifth seed Elena Rybakina secured a comprehensive 7-5 6-2 victory over Varvara Gracheva.
Rybakina is chasing a second major, having won Wimbledon in 2022 and finished as the Australian Open runner-up to Aryna Sabalenka in 2023.
Czech 19-year-old qualifier Nikola Bartunkova stunned Switzerland's Tokyo Olympics gold medallist Belinda Bencic, knocking out the 10th seed with a 6-3 0-6 6-4 victory.
Bencic, 28, had come into the Australian Open in strong form after winning all five of her singles matches for Switzerland at the United Cup earlier in January.
Maddison Inglis made a remarkable turnaround in the final minutes of a three-hour and 20-minute battle to beat Germany's Laura Siegemund in a tie-break.
The Australian qualifier won the topsy-turvy marathon affair 6-4 7-6 (7-3) 7-6 (10-7) to make it to the third round.
