Jennifer Lopez prepared for her Las Vegas residency "like a boxer training for a fight".
The 49-year-old singer-and-actress feels "much better" when she's in good shape but needed to push herself with more intense workouts in order to get through the long stage shows in Sin City.
Speaking to Closer magazine, she said: "I've always taken pretty good care of myself. I make sure I do intensive workouts at least three or four days a week, because I can see the results and I feel a lot better when I'm in shape.
"I also need to train very hard while doing my Las Vegas shows because I had to be extra fit to get through each performance.
"It's like you're a boxer training for a fight. But it really pays off and, as you get older, it's even more important to put in the extra hours of training to keep your body toned."
The 'On the Floor' hitmaker credits her dance background for being able to "push" herself hard when she needs to.
She explained: "I started out as a dancer and I remember whenever I arrived at the gym in the morning, I would spend as many hours as I could there.
"I would train until the point of exhaustion. So I have the mentality to be able to push myself pretty hard."
Jennifer had a crisis of confidence after her first flush of success when she received a backlash and wave of criticism.
She said: "After I had had some success, I suddenly saw a lot of criticism being thrown my way and it made me start to doubt myself.
"There was all this talk that I wasn't really an actress, wasn't really a singer, and after a while that stuff starts to wear you down.
"I'm very sensitive and it was hard for me to avoid focusing on the negative things that were out there instead of looking at the positive.
"You lose faith in yourself. I told myself, 'You know that you have the talent and the drive to keep being successful, that you have a lot of fans out there who love and support you.' So I started believing in myself again."