User blog comment:Xilinoc/Ch. 562 - THE VILLAIN 2 (Mask De Masculine is a Mexican luchador in a German army)/@comment-6963992-20131218213812

Renji defeated mask physically and since we haven't seen an upper limit on how much stronger a shinigami can get I don't see why back breaking training from people who's strength = several captains and hundreds of officers themselves wouldn't pay off to making him stronger than or as strong as a captain. Besides Rose and Kensei didn't fight him at the same time, nor did they know all of his abilities.