Wildcard Walton stayed cool at the height of the Paris heatwave on Tuesday to shock a frazzled sixth seed Daniil Medvedev, "hanging tough" in the final set to come from behind and beat the former world No.1 6-2 1-6 6-1 1-6 6-4.
Then when the evening came and temperatures mercifully dropped, the unsung Birrell felt inspired by her friend to somehow hit an even hotter comeback trail under the lights to outplay American world No.5 Jessica Pegula 1-6 6-3 6-3.
If six Aussies through to the second round wasn't good enough news, main man Alex de Minaur was given a walkover into the third round following the withdrawal of his dangerous second-round opponent Alexander Blockx with a right ankle sprain.
"As Aussies we really do cheer each other on,'' Birrell said.
"I'm good friends with Adam, and his coach has helped me out in my career.
"I was following his match and when I saw he'd won, I saw them and was super happy for him.
"I did feel inspired by his win. When you see someone have a result like that, it does give you a little bit of a pep in your step."
Stunning the former world No.1 for the second time in nine months, Walton took advantage of one of Medvedev's unfathomable Jekyll and Hyde days, winning in three hours 22 minutes on Court Suzanne Lenglen.
"It's huge," Walton declared, clutching his head in his hands in near disbelief at repeating his previous career-best triumph over Medvedev at the Cincinnati Open last August, but now on a grander stage.
"That win in Cincinnati definitely gave me the belief. I hung tough, I believed and I'm really excited. To get a first top-10 win at a slam is pretty epic."
Walton reckoned that after four curiously uneven stanzas, he was "proud" of rallying in the fifth set from 3-1 down when not knowing "which version" of Medvedev was going to materialise.
The world No.97 felt the key was saving two break points with the Russian on the verge of going 4-1 up.
"I knew I just had to fight," he shrugged.
Fight he did. At 4-2 down, Walton won the next two games, then survived three break points at 5-5 before the Russian cracked, double-faulting on the way to being broken to love.
Walton reckoned the 32C temperatures certainly helped, but it appeared initially to make Medvedev a Russian bear with a sore head.
Medvedev's wife Daria, sitting in the players' box, scolded him: "It's hot for everybody. Everybody is suffering. You need to behave!"
He did - but he never looked comfortable on the surface with which he's long had a love-hate relationship, and Walton ended up with a win he's sure will be big news back in his little rural hometown of Home Hill.
"It's great, just a pretty cool story to have been brought up in Home Hill and then moved to Brisbane, then to America (as a college player), and now playing pro. It's a pretty special story," Walton said.
So was Birrell's as she came back from a fearful pounding early in her evening match on Court Simmone Mathieu, Roland Garros's picturesque garden court, only to bloom spectacularly as Pegula surprisingly wilted.
To start with, it really did look like the world No.5 against the No.83 as Pegula made the 28-year-old feel "demoralised" by a ceaseless stream of winners.
But cheered on by her "Paris family", the friends of her mum with whom she stays during the tournament, Birrell, whose crisp, clean hitting increasingly discomfited the 2024 US Open finalist, reckoned a turning point was the perfect audacious drop shot that earned her the key second-set break.
Birrell, the 28-year-old German-born former Aussie No.1 who had lost her previous four matches and all three she's played on clay this year, then forged into a 5-2 lead as Pegula's late fightback fell short.
The day's one big disappointment was that hopes of a domestic second-round clash between Walton and Alexei Popyrin evaporated with the Aussie No.2 cutting a dejected figure after capitulating 3-6 6-3 7-6 (7-3) 7-5 to American Zachary Svajda.
