Goalkeeper Craig Gordon also played a starring role with two penalty saves as the Jambos fought back to reach the quarter-finals of the Scottish Cup with a 4-2 shootout victory over St Mirren on a tense night at SMISA Stadium.
The Edinburgh side looked on course to exit the competition on Monday as they trailed to a first-half goal from Mikael Mandron until Nieuwenhof, who had not played since March 3 last year due to a hamstring problem, stepped off the bench in the 84th minute to equalise within seconds of entering the fray.
"He's got this ability to drift into the box, and it's no coincidence that the ball dropped for him. I'm made up for him because he's been out for a long time and he's worked ever so hard," Hearts boss Neil Critchley said of Nieuwenhof, who joined in July 2023 after A-League Men stints with Sydney FC and Western Sydney.
"That's why we put him on as well, because he's shown that in a game recently that we played behind closed doors."
With the tie still deadlocked at 1-1 at the end of extra-time, Critchley's resurgent side - unbeaten since Boxing Day - then secured their progress in the shootout as 42-year-old goalkeeper Gordon saved the Buddies' first two kicks.
James Penrice sealed the victory after Gerald Taylor had seen his kick - Hearts' fourth - saved by Zach Hemming.
"Obviously, when it goes to penalties, you know the cliche that it's a lottery, but there's also a lot of skill involved," Critchley added.
"We practiced penalties yesterday. I'm really delighted for the ones who stepped up and scored. Craigie's pulled off a couple of fantastic saves."
Saints, who had lost five of their previous seven matches, broke the deadlock in the 22nd minute when Mandron stabbed home from just inside the six-yard box after Mark O'Hara's low curling shot was parried out by Gordon.
Hearts offered little in response for the remainder of the first half and were booed off by their 1600 travelling supporters at the break.
The Tynecastle side then lost Lewis Neilson and Socceroos midfielder Cameron Devlin to injuries, with January signings Michael Steinwender and Sander Kartum entering the fray to help salvage the tie.Â
But it was another substitute who eventually got Hearts on level terms in the 84th minute when Nieuwenhof was perfectly placed to drill home from 10 yards out after Penrice's attempt to get a shot away from James Wilson's low cross broke into his path off Phillips.
SCOTTISH CUP QUARTER-FINAL DRAW:
Aberdeen v Queen's Park
Celtic v Hibernian
Hearts v Dundee
Livingston v St Johnstone
Ties to be played on the weekend of March 8-9.
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