Two days after they pulled off major shocks with the performances of their lives, Gold Coast's Birrell downing women's fifth seed Jessica Pegula and Walton knocking out men's sixth seed Daniil Mevdevev, the follow-ups on Thursday fell horribly flat against opponents who looked eminently more beatable.
But with the mercury hitting 30 and her Ukrainian opponent appearing to be flagging in the heat, Birrell was initially bamboozled and eventually outlasted by unorthodox Oleksandra Oliynykova, succumbing to 'double trouble' as her serving faltered and she folded in a 10-point tiebreak decider 6-3 0-6 7-6 [10-5].
Soon afterwards, Walton, whose victory over former world No.1 Medvedev had a potentially career-changing feel, was largely outclassed by American Zachary Svajda, who'd beaten Alexei Popyrin in the previous round and again proved the Aussie slayer in a 6-3 6-4 6-7 (4-7) 6-2 win.
The double setback left the Australian challenge down to just two singles players, with Daria Kasatkina set for a second-round match against Swiss qualifier Susan Bandecchi later on Thursday and Alex de Minaur to face Jakub Mensik in the men's third-round on Friday.
Just as against Pegula, the 28-year-old Gold Coast fighter Birrell produced a stirring fightback but she found the all-court variety of the crafty Oliynykova a much different challenge, a puzzle which she eventually couldn't solve.
After looking a little bewildered in the opening stanza, it appeared she'd cracked the code, latching onto the moon balls, drop shots and loopy slices to whitewash the increasingly weary-looking Ukrainian in the second.
But in the decider, Oliynykova, who at times appeared to have been wilting, somehow found fresh reserves and in a thrilling 10-point tiebreak decider, her rhythm-breaking game proved too much for Birrell as she prevailed 6-3 0-6 7-6 (10-5).
In the breaker, 28-year-old Birrell threw in two double faults as the pressure increased, taking her overall tally to 11, which proved hugely costly as she coughed up 43 unforced errors all told.
Oliynykova's quirky but effective game was illustrated on match point when she delivered an underarm serve to initiate a point which ended with Birrell smashing a backhand drive volley into the net after nearly two-and-a-quarter hours of fascinating duelling.
It led to more emotional scenes as the heavily tattooed Oliynykova, who really introduced herself at the Australian Open in January when she turned up with a star-festooned face and gave defending champion Madison Keys a scare, went over to her father, who's on leave from the Ukrainian army, to embrace.
After her previous round win, the outspoken Oliynykova had launched a withering attack on Russian players who've kept silent over the Ukraine war and who she believes are part of the country's propaganda machine .
Asked about the prospect that Russian and Belarusian players might soon be playing under their national flags again, she had said: "This flag is a symbol of terror, something that the Russian soldiers are flying after they are destroying the city completely.Â
"In the modern context, it's the same thing like using, I don't know, a swastika. Let's go on court with a swastika. The fact that now this is under any discussion, it's horrible."
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