World No.6 Tsitsipas pulled out of his head to head with Nicolas Jarry with a back issue after teammate Maria Sakkari brushed aside Daniela Seguel 6-0 6-1 in the morning session at Sydney's Ken Rosewall Arena.
But another Stefanos - towering 19-year-old Stefanos Sakellaridis - gave Jarry, the world No.19, one hell of a scare.
Stefanos Sakellaridis made a bold fist of replacing Stefanos Tsitsipas in the men's singles match. (Steven Markham/AAP PHOTOS)
Despite a near 400-place gulf in the rankings, Sakellaridis fought back from a set down and was only broken by the Chilean in a match-deciding third set.
"That was an incredible match and Stefanos played amazingly," said Jarry, who triumphed 6-3 3-6 7-5 to square the tie at 1-1 heading into the mixed doubles.
"He was a very tough opponent and I'm happy to be there, mentally you've got to go a level up in this type of event."
Tsitsipas - runner-up at last year's Australian Open - was a keen observer of Sakellaridis's gallant effort after withdrawing from the singles match at the eleventh hour.
The 25-year-old Tsitsipas intended to partner Sakkari in the mixed doubles after she made light work of Seguel.
The Chilean showed some resistance early in the second set after Sakkari wrapped up the first in 26 minutes.
But the gulf between the world No.571 and Sakkari was abundantly clear as the Greek kept her foot on the gas to claim a 6-0 6-1 victory.
Maria Sakkari celebrates her opening win in the United Cup. (Steven Markham/AAP PHOTOS)
"I don't feel match sharp but I feel good," Sakkari said.
"I'm very happy to have given my team the first point and I have full belief in the team so I'm super excited for what's coming."
The last nation to kick off their United Cup campaign, Greece are in Group B, also with Canada, who defeated Chile 2-1 in their opening clash.
Later on Tuesday, Novak Djokovic will spearhead Serbia's charge towards the quarter-finals against Czechia in Perth.
A win in the final Group E tie at RAC Arena would put Team Serbia on collision course with Australia in the last eight after the host nation booked their quarter-final spot with a gutsy win over the US on Monday.
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