With neither Ash Barty or Ajla Tomljanovic available it fell to Daria Gavrilova and Storm Sanders to carry the fight to a Belgian team which had already beaten Belarus 2-1 in the group's opening tie.
Their successes mean victory against Belarus in the final group tie on Thursday will put Australia into the semi-finals.
Gavrilova marked her return from a nine-month lay-off with a gutsy display in which she had to take a medical timeout before beating Greet Minnen 6-4 1-6 6-4.
"I'm exhausted but I'm really excited," the 27-year-old said.
"I was a bit scrappy at times but my fighting spirit never went away so I'm really happy."
For Sanders this was her first win over a top-20 player.
"It feels amazing," she said. "I knew it was a tough ask playing Mertens but we had a good gameplan, I went out and executed and even though I lost the first set I felt like I was still in control."
Gavrilova hadn't played since exiting the Australian Open in a loss to Barty and her return to action loomed as a baptism of fire.
With Barty, a member of the side that reached the final in 2019 staying in Australia to recover from a busy season and prepare for the Australian Open, and Tomljanovic forced to withdraw on game day due to a non-Covid-related illness, it was fair to say Australian hopes were low when Gavrilova took to the court.
The Russian-born player underwent Achilles surgery earlier this year, sending her world ranking sub-400, and the challenge against world No.70 Minnen shaped as a bridge too far.
But Gavrilova showed little signs of rust early on as she broke the Belgian in the first before holding serve.
The pair then broke each other with Gavrilova eventually claiming the set. Minnen, however, rallied to wrap up the second set in 32 minutes.
Gavrilova's serve was noticeably weaker and she lacked the fluidity that had given her the early lead.
After a medical timeout she returned with heavy strapping on her left thigh and the break seemed to have done her a world of good.
Her zip in service returned and at 5-4 she broke Minnen to take the match and put Australia 1-0 up.
Sanders, the world No.33, then faced Mertens, 15 places her superior.
Sanders more than matched Mertens in the opening stages with the pair going toe to toe.
But just as the Australian was settling into her rhythm, she dropped four straight points on her own serve to give Mertens an opening she seized, holding her own serve before breaking Sanders again to take the set 3-6.
The second, after a flurry of breaks mid-set, went to a tie-break.
Sanders pounced to take a convincing 5-1 lead, then slipped to 5-4 before recovering to force two set points. She double-faulted on the first, but won the second against serve to seal the breaker.
With momentum behind her Sanders then played inspired tennis to run away with the third set 6-0 winning 12 of the last 13 points for an unexpected triumph.
"I don't think I have played a set that well in my life," she said. "I don't think I made an unforced error. I was just enjoying it."
In the other daytime match Russia brought Canada back to earth after they had upset defending champion France on Monday.
Daria Kasatkina eased past Carol Zhao in straight sets, then Anastasia Pavlyuchenkova clinched the win overcoming Rebecca Marino 6-4 4-6 6-2.
Russia's dominance was completed in the doubles where Veronika Kudermetova and Liudmila Samsonova defeated Gabriela Dabrowski and Marino 6-3 6-1.
Russia meet France on Wednesday with semi-final place at stake.
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