The effects were evident as Swiatek's level dipped in the second set of her 7-5, 6-1 defeat, sealing her earliest exit from Paris since her debut campaign in 2019.
It was a 25th birthday to forget for Swiatek, who has not lifted a clay-court title since her 2024 French Open triumph, and the Pole was clear why she was beaten comprehensively by 15th seed Kostyuk.
"I lost control of the match, and there was no way to come back, because I felt worse and worse. This isn't positive, and this is different than losing to Elina Svitolina in Rome or to Mirra Andreeva in Stuttgart," Swiatek told reporters.
"It's not great. I know I lost because I was tense, and my body couldn't do... the proper things, but it's not the first time, as well. So yeah, I just need to work on it.
"Honestly, maybe the toughest loss is when you had the match in your hands, you made stupid decisions, you let it go, and your opponent suddenly came back.
"It's also bad when your tennis is terrible, and you know you were worse than the players you play with. But I feel I lost today because Marta used the opportunity, and I was super tense."
Season to improve?
Swiatek said she was fully aware of what she needs to work on to improve and would not rush it.
"Maybe it's not going to take one week or one month," the six-time Grand Slam champion added.
"Maybe it's going to take even a season or something, but I need to believe that I can work through this and not be thrown off so quickly."
Swiatek will need to dust herself off quickly ahead of the grass-court season as she bids to defend her Wimbledon title when the tournament begins on June 29th.
"I haven't thought about grass at all," Swiatek said.
"Yeah it was a great year last year, but also, my previous results weren't good. They were good, but depending on how you look at it.
"I'm going to probably take it easy and try to be patient and be humble. I don't know how it's going to be. I'm not a wizard."
