(Las Vegas, NV) -- Undefeated welterweight boxing champion Floyd Mayweather Junior puts his perfect record on the line tomorrow night when he faces MMA star Conor McGregor in a boxing match in Las Vegas. Mayweather is the heavy favorite as this is McGregor's first professional boxing match. McGregor is 21-and-3 in the UFC. The fight will air on Showtime pay-per-view. The main card begins at 6 p.m.