Today, he is at peace with himself and has rediscovered happiness on and off the pitch in Frankfurt, where he has become a regular at right back and was voted Best Player of the Season last year in Eintracht Frankfurt.
But before that, he went through a mentally stressful period after he left FC Midtjylland as a 20-year-old, which threatened to end his career. At 27, he wanted to return to Denmark and live "a normal life" outside the parameters of professional football, where the constant pressures of delivering the goods took away his joy of the game.
"I had lost the desire to play football and lost the desire to be part of that circus. Because I had lost the desire for that whole world – the whole mentality, the whole business. I had lost the joy of training and of playing", says Nissen Kristensen to TV2 Sport.
Kristensen lived in a shadowy existence, characterized by inner turmoil, nervousness, and a fundamental uncertainty in relation to the matches and the constant pressure that comes with life as a professional footballer.
"It became a kind of masquerade – a mask I put on every day – and in the long run it became too hard for me. That mask has actually been a part of me throughout my time abroad", says Nissen Kristensen, who hit bottom three years ago.
"My year in the Premier League – with the relegation with Leeds and the enormous pressure that comes with it – I was not ready for. It's the biggest league in the world with the biggest media attention in the world, and I couldn't handle that."
"When the dream of having a professional football career came true, it was just not at all what I had imagined. All of that together wore off. And I also felt bad mentally during my last period at Leeds."
Rasmus Nissen Kristensen, therefore, left the English club after just one season and joined AS Roma. But it did not turn out as he had hoped for, even though he was often given chances to shine.
"Even though I got better at handling things, I just never managed to get into the plus in Rome. And when the EURO in Germany arrived, and I spent the entire tournament on the bench, I thought: "I simply can't take another year like this".
Nissen Kristensen eventually recovered and found the right environment when his girlfriend persuaded him to join Eintracht Frankfurt instead of going home, and he has since become a pillar in defense at Frankfurt and in the Danish national team.
"I am done chasing all kinds of things, and I am much relaxed now", says the Danish international.