Keisuke Honda became the first Japanese player to score at three World Cups when he netted a vital equaliser in the 2-2 draw against Senegal, which ultimately proved decisive in Japan's quest to qualify for the last 16.
But Mertens says the Red Devils are wary of a possible repeat of their Euro 2016 quarter-final, when they crashed to a 3-1 defeat to Wales despite being favoured to progress. Yet another tournament at which the Red Devils flattered to deceive.
Noting Japan had dramatically changed their coach since last year's friendly - Akira Nishino was brought in just two months ago - he praised the side which beat group winners Colombia in their opening match.
But could this squad - featuring several Premier League winners - finally be about to deliver?
Belgium's forward Eden Hazard (centre) fights for the ball with Japan's midfielder Makoto Hasebe (left) and midfielder Gaku Shibasaki during their 2018 World Cup round of 16 match at the Rostov Arena in Rostov-On-Don on July 2, 2018.
Belgium 3-2 Japan with goals from Haraguchi, Inui, Vertonghen, Fellaini and Chadli.
Thibaut Courtois saved the attempt, and Belgium broke from the corner that followed to snatch the victor and book a last-eight tie with Brazil.
However, he refused to look past Japan towards a potential quarter-final against Mexico or Brazil.
"I think they are doing well", said the Napoli player.
"That now gives them the belief and it's a powerful thing to have". "It tells you everything about this group of players", said Spaniard Martinez. "You have to do something and find a way and boy did he do it".
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But Martinez would only concentrate on the positives after their remarkable win on Monday. "So, I don't forget it", the 31-year-old said. "It was about getting through, and we did that".
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"This is for everyone in Belgium".
"Keep believing in Belgium".
"I thought this is a World Cup, and we were faced with the strength in depth of Belgium, that's how I felt at the end of game".
Belgium entered the tournament as one of the favorites.
With Japanese facing a fourth straight loss in a World Cup, Takayuki Suzuki latched on to Shinji Ono's pass to slot home the equaliser for the Japanese. "Is this the sign of a champion, a comeback like that?"
And unlike teams like Argentina that struggled to find chemistry, Belgium have been a consistent unit throughout the past two weeks.
After making nine changes for last Thursday's win over England, Martinez is expected to revert to the line-up which impressed in one-sided wins over Panama and Tunisia. "We have top players and we play as a group, especially after the win over England", said the Chelsea star.
"Since it's a temple and the game is taking place quite early in the morning, we hope people will watch the game calmly without whistles or drums, except when goals are scored", Ishida said.