In tricky conditions created by a brisk wind in the Californian desert, Raducanu looked uncomfortable throughout as Uchijima adapted impressively.
The 2021 US Open champion made a host of errors to allow Uchijima, a tenacious opponent who continued to put balls back into court, to come out on top of the often scrappy rallies.
Raducanu faced break points in her opening six service games, a result of the pressure created by a lack of accuracy as she looked to be aggressive.
Five of those games ended in Raducanu losing serve as she trailed by a set and a break to the 52nd-ranked Uchijima.
The Briton cut a subdued figure after she could not convert any of three chances to break back for 3-3.
After Uchijima dug deep to hold, Raducanu did not win another point and trudged off court having lost for a sixth time in her nine matches this year.
Raducanu was watched from the stands by Slovakian coach Vladimir Platenik, who has linked up with the British number two on a trial basis.
Platenik, 49, was coaching New Zealand qualifier Lulu Sun when she beat Raducanu at Wimbledon last year, while he has also worked with top-10 players Dominika Cibulkova, Daria Kasatkina and Veronika Kudermetova.
When asked which areas of her game she wanted to work on with Platenik, Raducanu smiled and said “everything”.
“I’ll need to improve starting the point. That’s a big part of tennis. I can do that a lot better,” she added.
“I could improve being more aggressive and having a better quality of shot going forward into the court.
“I could improve every area of my game after today’s performance.”
Shortly after Raducanu’s exit, British number three Sonay Kartal did move into the second round, making the most of being given a ‘lucky loser’ spot having been beaten in qualifying.
Kartal, 23, won 7-6 (8-6) 6-2 against 38-year-old American qualifier Varvara Lepchenko and faces Brazilian 16th seed Beatriz Haddad Maia next.
In the men’s draw, Jacob Fearnley was unable to set up an all-British clash with Jack Draper after losing to rising star Joao Fonseca.
