The 100th-ranked Cristian made it to her first final as a “lucky loser.”
She lost in qualifying last Sunday but still entered the main draw after fellow Romanian Sorana Cirstea withdrew from the event with a calf injury.
Cristian further benefited on her way to the final from two opponents withdrawing with injuries: Rebecca Peterson of Sweden quit their last-16 match during the first set, and Simona Halep pulled out before their semifinal.
Cristian and Riske swapped breaks early in the opening set of the final before the Romanian gained control with dominating groundstrokes to win four straight games from 2-2.
Riske avoided break points and had nine winners in the second set to level the match.
The American was a break up in the decider but lost her advantage in a nine-minute service game at 4-3, and missed four break chances in the next.
She broke Cristian at 5-5 and closed out the win when the Romanian hit a backhand long.