This time last year, Boutler was a seeded player at Flushing Meadows but is now in danger of slipping out of the world’s top 50 after a difficult season.
The powerful baseliner has never reached the second week of a major and seemed to lack confidence at times against Kostyuk – seemingly systematic of her lack of her form.
Despite only winning once in her past eight matches, Boulter said she had regained her “buzz”.
Boulter believed she had found a new perspective after taking “days off away from the sport” following the Canadian Open last month.
“You can look at the results and think ‘she’s having a tough time’, but I’m actually really excited,” she said.
“I lost it a couple of weeks ago in Montreal and I’d say that is a pivotal moment in my career.
“It was a full reset in the middle of the US hard-court swing and I think I’m in a good place to move forward.”
Saving five match points as she served at 5-3 illustrated how Boulter will not stop fighting and helped her create three break points in the 10th game.
However, Kostyuk recovered to serve out the match and move into the second round.
Boulter coped fine with the hotter conditions and put her struggles down to the tweaks she has been trying to make to her serve.
“I’m trying to find rhythm and don’t fully commit to the serve I’m going for,” added Boulter, who was broken four times.
“At this level, putting the serve in the court is not going to get it done.
“Otherwise, on the court, I matched Marta well and returned decently well. Considering my form it was a relatively competitive match.”