Lowly ranked Raphael Collignon overcame severe cramping to upset de Minaur 7-5 3-6 6-3 in a three-hour, 12-minute epic at Ken Rosewall Arena on Saturday.
Two-time Cup-winning great Todd Woodbridge was furious  officials didn't call for a physio to treat Collignon when he was writhing in agony on the court midway through the opening game of the deciding set.
"This is ridiculous," Woodbridge said in commentary for the Nine Network.
"This is so, so cruel. Get the physio out there now. This is more important (than the result). This is a health issue now."
Both the tournament doctor and referee came to the court to assist the 23-year-old, but Woodbridge said that was "not enough".
"I've just sent a couple of messages just trying to clarify what the ruling is here because I thought the understanding was that we got rid of the barbaric look of what we've just seen," the doubles legend said.
Not allowed a medical time-out because cramping is deemed to be the result of a "lack of conditioning", Collignon took his cap off and appeared to be walking to the umpire's chair to shake hands during that first game of the third set.
But the world No.91 changed his mind, played on and heroically battled to the biggest win of his career.
Playing for his country and riding high two weeks after taking down dual grand slam finalist and world No.12 Casper Ruud at the US Open, it was understandable why Collignon did not wish to surrender.
He came out blazing from the get-go, refusing to be intimidated by Australia's world No.8.
After being broken to fall behind 6-5, de Minaur fended off four set points but not a fifth in an epic 12th game as Collignon took the marathon first set in 86 minutes.
Facing the grim prospect of leaving Australia trailing 1-0 in the best-of-five-match tie before Jordan Thompson took on the higher-ranked Zizou Bergs in the second singles rubber, de Minaur raised his game in the second set.
He charged to a 5-1 lead, only for Collignon to somehow muster another break as the cramping began to take hold.
But the home-city favourite was unable to shake his dogged opponent, with the underdog digging deep to give Belgium a 1-0 lead in the tie.
Thompson must beat Bergs to avoid Australia needing to win both of Sunday's reverse singles matches and the doubles to advance to this year's quarter-finals in Italy in November.
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