The dogged Australian prevailed 4-6 6-3 7-6 (8-6) on Tuesday, winning the final four points of the contest to snatch a round-one win from under the Spaniard's nose.
Wildcard Jason Kubler followed in his fellow Queenslander's footsteps in the evening, ousting Argentinian qualifier Tomas Martin Etcheverry 7-6 (7-2) 7-5.
Millman, who qualified for the Adelaide event, is rallying after a bizarre eye injury contributed to a battling 2022.
Playing in Mexico last February a ball deflected off his racquet in between points and struck him in the eye, scratching his cornea.
Ranked 80th in the world at the time, the man who beat Roger Federer at the 2018 US Open finished the year down at No.147 in the world.
"Last year was pretty average for me; I feel refreshed," Millman said.
"Back-to-back matches in the last couple of days; you've just got to get that winning feeling and it's not easy to come by here, even in qualies.
"I'll take that momentum any day of the week."
A see-sawing final set saw Millman down an early break then up 4-2, only to relinquish his advantage and then play fearlessly with the match on the line.
Millman hit just two winners in a tame first set but opened up in the second with 11, then ventured to the net in the third set in a move that turned the tables on the world No.38.
"Even at 33 I'm still trying to add little bits to my game," Millman said.
"Obviously it's (approaching the net) something that could have won me a few more matches early in my career."
Millman has set up a second round match with seventh seed Alejandro Davidovich Fokina, while Kubler will play sixth seeded Serbian Miomir Kecmanovic.
Kubler, whose impressive wins at the recent United Cup have catapulted him to a career-high world No.86, turned on the power against Etcheverry.
He also had to overcome a power outage that briefly cut the lights midway through the second set.
Both Millman and Kubler were earlier this month granted wildcards for the Australian Open starting next week.
It wasn't a good day for Australia's world No.79 Chris O'Connell, who fell 6-4 7-5 to surging American Tommy Paul.
O'Connell, a lucky loser entrant into the main draw after losing to Millman in qualifying, went toe-to-toe with the world No.35 in the 90-minute scrap.
Paul pounced at the crucial moments, breaking O'Connell at 4-5 to claim the first set and repeating the dose at 5-6 after an equally evenly-poised second set.
O'Connell will switch focus to the Australian Open, the much-improved Sydney product now entrenched in the top 100 after surging about 400 places to sit just outside of the mark in 2019.
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