Two-time Australian Open champion Naomi Osaka's Grand Slam comeback ended abruptly on Monday, crashing out to Caroline Garcia, but US Open winner Coco Gauff and men's third seed Daniil Medvedev surged on in Melbourne.
Order of Play | Watch Live on DStv
On a baking-hot day at Melbourne Park, men's seventh seed Stefanos Tsitsipas and women's sixth seed Ons Jabeur also tasted first-round victory.
But Wimbledon champion Marketa Vondrousova and veteran Andy Murray made early exits.
Japanese star Osaka, 26, stepped away from the sport in September 2022, citing mental health concerns.
After giving birth to a baby girl in July, she decided to return for the new season and was back at the scene of her title-winning exploits in 2019 and 2021.
But she was unable to rediscover her championship-winning form against 16th-seeded Frenchwoman Garcia after just two competitive games in Brisbane since returning.
"She's been through a lot but I'm just very glad to see her back and I hope she can enjoy the tennis now and have fun out there. She's a great girl," said Garcia.
"I mean, six months after giving birth she is playing quite amazing already, so we have to watch out."
Medvedev battled tough conditions on Margaret Court Arena against French qualifier Terence Atmane, who smashed his racquet, served underarm and retired in tears after suffering bad cramping that forced him off court while losing 5-7, 6-2, 6-4, 1-0.
"When I started to feel tough physically he started cramping, so it was brutal conditions," said Russia's Medvedev, a finalist in 2021 and 2022.
"I was getting tired but many times in these situations the other guy is getting tired too and it's about who manages it better."
The Russian will face Finland's unseeded Emil Ruusuvuori in the next round.
Tsitsipas, who lost last year's final to Novak Djokovic, was stunned by Belgian lucky loser Zizou Bergs in the first set, but woke up to seize complete control and win 5-7, 6-1, 6-1, 6-3.
But the tournament is over for five-time finalist Murray, possibly for the last time.
The 36-year-old British player crashed out to Argentine 30th seed Tomas Martin Etcheverry in three sets, admitting that "not much went right".
"It's a definite possibility that will be the last time I play here," he said.
Women's fourth seed Gauff said ahead of the opening Grand Slam of the year that she wanted to win "multiple" major titles after her breakthrough at the US Open in 2023.
The 19-year-old put the plan into motion with a 6-3, 6-0 rout of Slovakia's 68th-ranked Anna Karolina Schmiedlova on centre court.
'DIDN'T PLAY GOOD'
"I was a little bit nervous coming in today. Probably could tell. I was able to just calm down and then play good, not my best, but good tennis," said Gauff, who successfully defended her Auckland title in the lead-up.
Her reward is a clash with unseeded American Caroline Dolehide in round two.
While she lived to fight another day, it was curtains for Vondrousova, who slumped 6-1, 6-2 to Ukrainian qualifier Dayana Yastremska.
The Czech seventh seed pulled out of the warm-up Adelaide International last week with a hip injury and said she was also struggling with a sore shoulder.
"She played a good match, I didn't play good, that's why (I lost)," she said after becoming the highest seed to depart so far.
World number 93 Yastremska, whose best showing at Melbourne Park is a third-round appearance in 2019, sported the slogan "Be brave like Ukraine" on her bag and said she wanted to keep the war in her homeland in the spotlight.
"I'm very proud of Ukraine, proud of the people, proud of the warriors and the civilians," she said, after setting up a clash with unseeded Frenchwoman Varvara Gracheva.
Another Ukrainian, Elina Svitolina, also progressed, demolishing Australian Taylah Preston for the loss of just four games.
Tunisia's Jabeur, a three-time Grand Slam runner-up, similarly only dropped four games in crushing qualifier Yuliia Starodubtseva.