Who would win in a MMA fight between Justin Bieber and Tom Cruise?

Bieber recently challenged Cruise to an MMA fight via twitter (apparently some publicity stunt).
Who would win if this happened?

Cruise is over 50 but has always been in great shape and has done fight training for many of his movies and he does pretty much all his own stunts.

Bieber is in his 20's and appears to be in good shape, I'm unaware he's had any fighting training.
Who would win in a MMA fight between Justin Bieber and Tom Cruise?
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Who would win in a MMA fight between Justin Bieber and Tom Cruise?
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