Murray was 10 and Monfils 11 when they met in the semi-finals of a junior tournament, the Frenchman winning before losing to Murray's brother Jamie in the final.
"He used to play with glasses," said the Wimbledon champion. "He had shaved hair, but quite a high cut. He was the same as he is now. He was just a great athlete, moved unbelievably well, smiling on the court.
"He enjoyed playing in front of a crowd, even though it was a small crowd. When you're 10, 11 years old, playing in front of 40, 50 people feels like it's loads.
"He's just always been a great entertainer, and he's great for the sport."
The pair have been friends ever since those early days and if Murray had to pick one player to watch, it would probably be Monfils.
The Frenchman, who is eight months older, also has fond memories of matches against Murray.
He said: "We grew up pretty much together. It's always fun to play against him. He's the same, he didn't change at all."
Eighty miles down the road from Rouen, the pair will meet for a sixth time in the adult ranks with a much bigger prize at stake - a probable semi-final against Rafael Nadal.
Murray leads their head-to-head 3-2 but Monfils has won both matches in Paris, one at Roland Garros and one indoors at Bercy.
They have not faced each other for three and a half years, making this an even more intriguing clash.
"It's a tough match," said Murray. "I think in the grand slams he's played his best tennis here by far. He loves playing in front of a big crowd.
"He's a great athlete. Maybe the best we have had in tennis. It's going to be an exciting match. I'm sure there will be some fun rallies. There always is when I have played against him.
"We haven't played against each other for quite a while, so I'm looking forward to it."
Monfils grew up in Paris and is a folk hero at Roland Garros.
This is his fourth quarter-final in six years. Roger Federer beat him in two of them while, after defeating David Ferrer in 2008 to reach his so far only grand slam semi-final, Federer ended his run again.
It is no surprise Monfils said about meeting Murray: "I'm happy I'm not having to take on Federer."
Murray practised on Court Philippe Chatrier on Tuesday morning but was still waiting to find out whether he would play on Roland Garros' centre court for the first time time this tournament or on Suzanne Lenglen, where he has played three times.
With Nadal facing Ferrer in the other quarter-final in a rematch of last year's final, it was a difficult call for the organisers.
Wherever the match takes place, Murray knows he will have the vast majority of the crowd against him, and they are likely to be very noisy.
But that may actually help Murray, who relishes such situations and could not take the grin off his face when he stepped out to jeers against Richard Gasquet here two years ago.
He said: "Obviously it's difficult, but it's a great challenge. It's more like a Davis Cup atmosphere.
"It's an intense atmosphere but you can also feed off that as well.
"It's the quarter-finals of a slam. I don't care whether no one in the crowd wants me to win or everyone wants me to win. I will fight just as hard to try and get the right outcome."
In Murray's favour will be his record against Frenchman - the Scot having won his last 20 slam matches against them dating back to a loss to Jo-Wilfried Tsonga in the first round of the Australian Open in 2008.
The order of play was released late in the afternoon showing Murray and Monfils would be on Chatrier with Nadal and Ferrer relegated to Lenglen.