Boxer Joe Calzaghe is facing a huge test in Las Vegas this weekend as he takes on America's Bernard Hopkins.
The super-middleweight world champion hasn't been beaten in 44 fights but he's fighting Hopkins in the next division - as a light-heavyweight.
Calzaghe has had to make sure he weighs a certain amount, to allow him to fight in the light-heavyweight category.
Both Calzaghe and Hopkins, who's only been beaten four times in 53 fights, have held world titles for 10 years.
It's Calzaghe's first fight in America, but he's said he'll retire if he loses to Hopkins.
The current BBC Sports Personality of the Year has been waiting for ages for the chance to face Hopkins in the ring, as he thought to be the world's best at the 175lb (79kg) limit.