Sirius tried to dodge as many of Regulus' blows as he could, but after awhile he just gave up. If he was going to land any hits, he'd have to take a few himself. He's already made him bleed, now he just wanted to knock him down.
Sirius wasn't used to fighting like this without someone interfering. He didn't mind it. Maybe they just figured they weren't going to do anything but beat each other senseless.