User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-16281343-20150718161758/@comment-16281343-20150718162352

Rizevim tried to use a Phoenix Tear but Diehauser made it [Worthless]. From the fight against Riser, he now had a clear idea on the properties of the Phoenix Tears so he can nullify it.

That is the reason why of all people, he chose Riser to be his opponent. The previous explanations of his actions all point to "why Riser? He can still do what he did with another opponent."