The four-time Roland Garros champion fell to a shock exit in Paris on Sunday (31.05.26) as she was beaten in straight sets by Kostyuk in the fourth round clash and rued how she "lost control of the match" against the Ukrainian.
Swiatek said in her post-match press conference: "I lost control of the match and there was no way for me to come back because I felt worse and worse, you know.
"So this is not, like, positive, and this is different than losing to Elina [Svitolina] in Rome or to [Mirra] Andreeva, I guess, in Stuttgart.
"So, yeah, it's not great. I know that I lost because I was tense and my body couldn't really do what my body... the proper things, you know, but it's not the first time, as well.
"I just need to work on it. Honestly, like, for me, maybe the toughest loss is when you had the match in your hands, but I don't know, you made stupid decisions and you let it go and your opponent suddenly comes back.
"It's also bad when your tennis is just terrible and you know that you were worse than the players that you play with."
However, Swiatek praised Kostyuk for making the most of the opportunity she was presented with.
The world number three said: "I lost today because Marta used the opportunity and I was super tense. I feel like I can work on that, at least, and there is a reason and there is maybe a solution. Maybe it's not going to take one week or one month.
"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 now turn her attention to the defence of her Wimbledon title but admits it is "hard to say" how she will perform on grass this year after a surprise success at the All England Club in 2025.
The six-time Grand Slam winner said: "I don't know. I haven't thought about grass at all. Hard to say.
"Yeah, it was a great year last year but, also, my previous results were not good, you know. I mean, they were good but depending, you know, how you look at it.
"So I'm going to probably take it easy and try to be patient and be humble. Like, I don't know how it's going to be. I'm not a wizard."
