Beatriz Haddad Maia will contest her second final in a week when she faces Zhang Shuai at the Birmingham Classic.
China's Zhang fought back to beat Sorana Cirstea 4-6 6-1 7-6 (7-5).
Haddad Maia and Zhang won the doubles title together in Nottingham last week but will contest the Birmingham final against one another later on Sunday.
Haddad Maia, 26, who is on a nine-match unbeaten streak, fought for two hours and 16 minutes to beat 2019 Wimbledon champion Halep.
She twice came back from a break down in the opening set before winning four games in a row to close it out.