i can't choose only one!!
so i would say:
capriati def clijsters RG final
capriati def hingis aus open final in 2002, if martina had won, i'm sure she wouldn't heve retired
s.williams def clijsters semi final aus open 03
henin def clijsters berlin final 03, if kim had won, justine wouldn't have won roland garros
henin def myskina olympics, the same reason as JCF, in addition, henin didn't want to win for her country but for her whereas myskina wanted gold medal for russia