As the clinches were broken up quicker by the Ref Kovalev gassed quicker

[TOBYLEE1]

Unlike popular belief in these threads Kovalev did initiate a lot of clinches against Ward in the first fight. As the fight doesn't go his way you can see how he physically and mentally eroded away.

As Weeks was breaking the clinches quicker Kovalev didn't have the chances to rest like in the first fight and gassed out even quicker. It didn't help that Ward was more active in the beginning of the fight.

[SplitSecond]

Don't think so, kovalev was visibly doing a lot more in this fight, and obviously had the plan to fight in the clinches to keep ward from racking points with the pity pats, unlike the first fight which the gameplan consisted mainly of neutralization.

Ref had nothing to do with it, kovalev simply worked more, and him trying to do work on the inside gave ward more room to hit him with better shots inside too.