But years later, he has now learned to be applaud just how well the Australian No.1 has adapted to life on the surface that natural Spanish clay-courters like him used to consider their stronghold.
Indeed, Munar is so impressed by de Minaur's strides on the clay, he fancies he will have to raise his game to new levels to topple the ever-improving Sydneysider in their second-round clash at the French Open on Thursday.
"Hundred per cent, it's going to be a very tough one," said Munar, a feisty customer who had a testy run-in at the net with Thanasi Kokkinakis in Madrid last year when he accused the Australian of telling him to "shut up".
But the 27-year-old, known affectionately as 'Jimbo' for his combative Jimmy Connors-style outbursts, has nothing but genial words for the Alicante-based de Minaur, who he remembers playing in local tournaments in Spain when they were boys.
The Sydney-born De Minaur had moved to Spain when he was five with his Spanish mum and Uruguayan dad, before returning to Australia at 13 and then going back to Europe to start moulding his career in the Spanish hot bed of tennis in his late teens.
"Alex has been working very hard, and he's developing very good tennis on all the surfaces, improving a lot in the last couple of years on clay," said Munar, who's currently No.64 in the world, never having quite managed to crack the top 50.
"I remember him struggling a little on the clay at the beginning when he turned pro, but he's one of the best players in the world anywhere he's playing now.
"He's been having success in Monte Carlo, good matches in Rome and we saw that he beat Rafa (Nadal) in Barcelona, so of course he's improving all the time.
"It's my favourite surface and probably not his best, but still he's one of the best in the world and I have to be at the top of my level."
De Minaur, currently standing at a career-high No.8 in the world rankings, will certainly start hot favourite against a man he beat on clay in Madrid three years ago and also in a NextGen Finals semi-final in Milan in 2018 that Munar recalls as "very beautiful".
"I haven't practised with him lately but we know each other a lot, and we've known each other since we were kids because we had some circuits in Spain that we both were playing.
"Now he's one of the best, and for me it's a mountain to climb but I will try."
Not that Munar is averse to the idea of a bit of giant-killing. Last year, the 1.83m battler slayed the 2.08m John Isner on the American beanpole's hallowed turf.
"I beat Isner at Wimbledon. Imagine!" he beamed. "Anything is possible."
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