Queensland wildcard Walton stayed cool at the height of the Paris heatwave on Tuesday to shock a frazzled Daniil Medvedev, "hanging tough" in the final set to come from behind and win 6-2 1-6 6-1 1-6 6-4.
Then when the evening came and temperatures mercifully dropped, the unsung Birrell somehow hit an even hotter comeback trail to outplay American world No.5 Jessica Pegula 1-6 6-3 6-3 and ensure she was an even more surprising conqueror.
If six Aussies through to the second round wasn't good enough news, main man Alex de Minaur was also given a walkover to the third round following the withdrawal of his second-round opponent Alexander Blockx with a right ankle sprain.
Stunning the former world No.1 for the second time in nine months, Walton took advantage of Medvedev's Jekyll and Hyde days, winning in three hours 22 minutes on Court Suzanne Lenglen.
"It's huge," he declared, after 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 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 how he'd rallied 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.
Walton reckoned the 32C temperatures certainly helped. "We grew up in the heat," he said on court. "I really like it when it's hot in Paris."
Medvedev didn't look as though he did, even if he suggested the heat had nothing to do with his loss.
Still, he had begun the match like a bear with a sore head, grumpy about everything and leaving Walton to be amazed to effectively be gifted two breaks.
His wife, the normally quiet Daria, in the players' box, quickly scolded Medvedev: "It's hot for everybody. Everybody is suffering. You need to behave!"
He did - and in set two, pinned the Aussie back on the baseline relentlessly, only for Dr Jekyll to reappear in the third when his poor choice of drop shots saw Walton pounce.
The match twisted again with Medvedev''s superb fourth set but Walton hung in to record a win he hopes is 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 moved to America (as a college player), and now playing pro. It's a pretty special story."
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 raced through the first set in 24 minutes, but Birrell battled. As the former US Open finalist's form dipped, her's rose and she earned the key second-set break with a perfect drop shot.
Birrell, the 28-year-old German-born former Aussie No.1, who had lost her last four matches and all three that she's played on clay this year, then forged into a 5-2 lead in the decider. Repelling Pegula's late fightback she took the victory in one hour 45 minutes.
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.
