Raducanu's 2022 season has been blighted by different kinds of injuries, ranging from blisters to side strains and hip and back problem, and she was playing in her first semi-final since winning the US Open last year.
The trouble started for Raducanu midway through the second set in her first meeting against Latvia's world number 19 Ostapenko when she took an off-court medical timeout to get treatment for a left glute injury.
Raducanu, who has plummeted to 77th in the rankings from a career-high 10th in July, managed to return to court but won only a few points before conceding the match in the deciding set while trailing 4-6 6-3 3-0.
Ostapenko won the Korea Open title in 2017 - the same year she lifted her maiden Grand Slam trophy at Roland Garros - and will meet world number 24 Alexandrova next in Seoul.
Second seed Alexandrova advanced to her second final of the season by defeating Germany's Wimbledon semi-finalist Tatjana Maria (35) 6-2 6-4 earlier on Saturday.
