Justin Bieber is "tougher than he looks", a top boxer reportedly thinks.

The 18-year-old singer has struck up an unusual friendship with renowned fighter Floyd Mayweather, who recently began a jail sentence.

Justin helped Floyd carry his belts to the ring before his bout against Miguel Cotto in Las Vegas last month and also celebrated with him after the athlete's victory.

Floyd's fellow inmates at the Clark County Detention Center in Nevada where the boxer is serving 87 days for domestic violence have apparently been teasing him about the association. The sportsman is paying no attention to the taunts though.

"He's told them that Justin's a great kid and a lot tougher than he looks," a source told National Enquirer.

Rapper 50 Cent was also at the match last month and is good pals with Floyd. Those sharing a jail with the boxer apparently approve of that friendship.

"They think he should have let 50 Cent carry the [belts] for good luck and they want Floyd to talk to Fiddy about putting on a small concert for them," the insider added.