Four and a half years after becoming the first player in the Open era to win a Grand Slam from the qualifying rounds in New York, the tournament's top seed got off to a disastrous start in the Romanian city of Cluj, conceding the first eight games to her opponent.
Trailing 6-0, 2-0, Raducanu won her first game on her opponent's serve, before asking to have her blood pressure taken at the changeover.
Clearly not at her best, the 23-year-old nevertheless managed to level the match at two games all before conceding the last four and losing in just over an hour.
Cirstea, who at 35 has already said this will be her last season on tour, won the fourth title of her career, and first in her home country.
"Someone please pinch me, because this is a dream come true," Cirstea said after compatriot and former world number one Simona Halep presented her with the trophy.
"I still can't believe that I'm holding the trophy as champion here in Cluj. I am so, so happy."
The world number 36 previously picked up titles in Tashkent in 2008, Istanbul in 2021 and Cleveland in 2025.
It was not all doom and gloom for British tennis on Saturday, however, as Katie Boulter fought back from a set down to win the Ostrava Open.
The former world number 23 beat 124th-ranked German Tamara Korpatsch 5-7, 6-2, 6-1.