Hiroki didn't actually care whether Ryo was playing to her full potential or not. He was actually just goading her because balling the star batter was a good strategy that was actually working against the Kisaragi Boys' team. There's a chance that if Ryo didn't change her tactics, the Kisaragi Girls' team would have won and ended up making it to Koshien.
WMG / Princess Nine
Hiroki intentionally goaded Ryo into not balling him in order to win the game.
Izumi's relationship with Ryo was more of a sibling dynamic; she only fabricated her hatred/ rivalry with her to mask how she really felt