Krejcikova hasn’t dropped a set all week after dispatching Zhu Lin of China 6-3, 6-2 in the semifinals Saturday.
Ostapenko played her fourth consecutive three-set match when she beat fourth-seeded Anastasia Potapova 5-7, 6-2, 6-4.
In a matchup of former French Open champions, Ostapenko leads the head-to-head series against Krejcikova 4-2, including winning their most recent match last month in the Italian Open third round. But they have never met on grass.
By beating Zhu, the 12th-ranked Krejcikova will return to the top 10. She will be playing for her seventh singles title Sunday and second of the year, after winning in Dubai in February.
Ostapenko took more than two hours to put down Potapova, who needed six set points to clinch the first set. Ostapenko built a 4-0 lead in the second to even the match.
The Latvian player snatched the decisive break in the deciding set when she made four return winners to take Potapova’s service game at 3-3.
“It’s important to win these kind of matches when you’re sometimes not playing your best and you just fight for every point,” the 17th-ranked Ostapenko said.
In her first final in almost a year, Ostapenko is aiming for her sixth singles title.
Top seeds Krejcikova, Ostapenko to play for Birmingham title