Defending Australian Open champions Jannik Sinner and Madison Keys advanced in Melbourne as Brits Sonay Kartal and Katie Boulter bowed out.
World No 2 Sinner advanced to the second round after his French opponent Hugo Gaston retired after two sets while trailing 6-2 6-1.
Gaston was in tears after he told Sinner he could not continue, with the Italian second seed consoling him.
Sinner, who will next face Australian James Duckworth for a place in the third round, said: "I've put in many, many long days in the off-season trying to become a better tennis player.
"But at the end of the day the most important part is to go on court and to enjoy, no? It's very special to start the season in a night-session match here in a Grand Slam, the packed stadium, just trying to do your best."
Keys found herself 4-0 down in the first set before recovering to win.
The American claimed a long-sought-after maiden Grand Slam victory here 12 months ago, beating Iga Swiatek and Aryna Sabalenka on Rod Laver Arena, but returning to the same court against Ukrainian debutante Oleksandra Oliynykova threatened to turn into a nightmare.
Keys lost the opening four games, clawed her way back to a tie-break and had to save two set points before finally relaxing and pulling away to win 7-6 (8-6) 6-1.
"The moment they say, 'Ready, play', it kind of all hits you in a way that I don't think you can ever really explain to someone," said a relieved Keys afterwards.
"But, as nerve-racking and as stressful as that can be, I'm still reminding myself of just how few people get to be in that moment, and being able to walk out today and have the crowd be as welcoming as they were, I'll take the stress any day."
Although she could not pull off the upset, it was a moment of immense pride for 25-year-old Oliynykova, who made her mark for more than just her temporary facial tattoos.
She has risen rapidly up the rankings and revealed she takes inspiration from her father, who is fighting for Ukraine in the country's war with Russia.
"I'm so proud of him," she said. "Actually, this is something that is keeping me motivated even more. After he joined the military, I did progress for over 200 (ranking) positions, because I know it was his dream to see me on this court."
Unlike most of her compatriots, Oliynykova still lives and trains in Ukraine, with all the dangers that currently brings.
"During the preparations, I was hearing the explosions," she said. "There was a couple of massive attacks the last night I spent in Ukraine before the trip here.
"There was an explosion just near my home, and a drone hit the home just across the road. My apartment was literally shaking because of the explosion. In my apartment, I have no electricity, no water, no heat. Every opportunity to say about this, I believe it's very important."
Naomi Osaka made a fashion statement and quite an entrance to the Australian Open, wearing a wide-brim hat, veil and holding a white umbrella as she walked onto Rod Laver Arena for her first-round match.
The four-time Grand Slam champion was playing No 65-ranked Antonia Ruzic of Croatia in the last match on Rod Laver Arena.
She was very nearly upstaged by Antonia Ruzic on her 23rd birthday, the Croatian leading Osaka by a break in the deciding set only for the 16th seed to prevail 6-3 3-6 6-4.
Osaka said of her striking look: "Nike let me design this one, it's modelled after a jellyfish. I'm just so grateful I get to be able to do the things I love. It's really beautiful."
Anne Keothavong, speaking on TNT Sports: "She's making a statement isn't she. That is all you have to say. I have never seen anything quite like it. That is the ultimate walk-on."
Tim Henman: "I am speechless."
Emma Raducanu will carry British hopes alone into the second round of the women's singles at the Australian Open after Boulter was beaten by Swiss star Belinda Bencic.
Boulter has slumped to 113 in the rankings and only sneaked into the draw at the last minute - to find herself paired with arguably the most in-form player in the women's game.
The British No 4 at least rallied in the second set before slipping to a 6-0 7-5 defeat and joining Kartal in making a first-round exit.
"As I settled into the second, I can take confidence from the fact that I can definitely improve myself," said Boulter.
"I was right there with her at times. So there is some confidence in that. I think I just need to play a bunch of those matches and just get used to those tight moments again."
Kartal declared her performance the worst she had played in years after slumping to defeat.
There was no disgrace for the British No 2, who is ranked 66th, in losing to 31st seed Anna Kalinskaya, but it was the manner of her 7-6 (7-3) 6-1 defeat than rankled.
Kartal arrived in Melbourne on the back of a very narrow loss to Elina Svitolina in Auckland that seemed to have carried on her momentum from a strong end to last season.
She initially had the upper hand against Kalinskaya but ultimately paid for relying too much on the Russian's errors.
Kartal played a poor tie-break and her chances of a comeback evaporated as Kalinskaya opened up a 5-0 lead in the second set.
"Obviously very disappointed," said Kartal, who is still looking for her first victory at the tournament.
"For me, it was not my finest at all, even my base level was not really there today, which is something I rely on.
"I think, if I could go back out there now and play that match again, I'd play it a completely different way.
"Maybe she did raise her level but my level dipped to a level that hasn't been that low for years, probably.
"I ended last year feeling good and feel like I was training well. The warm-up this morning, I was playing well.
"I'm super disappointed, especially after Auckland. It's a long old season ahead so I'm not going to look too deep into this."
Tennis showman Gael Monfils bowed out of his 20th and final Australian Open with a mix of pain and regret as he fell to an injury-hampered 6-7 (3-7) 7-5 6-4 7-5 defeat in the first round to local qualifier Dane Sweeny.
The 39-year-old Frenchman had flashes of his trademark flamboyance and somehow made a game of it despite being severely restricted in his movement amid a heady atmosphere at Kia Arena.
"My journey started in 2003 with you guys, now we are in 2026 and somehow it's the finish line. Thank you so much for this amazing ride, you guys have been unbelievable," Monfils told the cheering fans who gave him a standing ovation.
"I've got a lot of great memories here. I've been very lucky to play here for many years. Thank you very much."
On a good day for the big names, Ben Shelton recorded a straight-sets victory, while men's fifth seed Lorenzo Musetti had fought back from a set down to lead Raphael Collignon by two sets to one when the Belgian became the latest player to retire injured.
Teenage star Joao Fonseca, though, is still waiting for his Grand Slam breakthrough after losing 6-4 2-6 6-1 6-2 to American Eliot Spizzirri.
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