Osaka, 21, was looking to win her third consecutive major before the loss, having won the 2018 U.S. Open and 2019 Australian Open.
"I didn't really care", Kenin said after her upset win, according to Eurosport's Tumaini Carayol.
Asked if nerves had played a part in a fairly miserable Paris experience, she said: "Yeah, I would say that, but at the same time I don't want to blame how I played on that".
Then she had to rally past her 83-ranked first-round foe, Russian Vitalia Diatchenko, 2-6, 6-1, 6-0.
"She has a great attitude".
The 23-time Grand Slam champion was second best throughout against the youngster, who had not even been born when the veteran first played this even in 1998. "She's such a tough player".
"I didn't care. I was, like, I'm going to check the mark", Kenin said of the negative response to her legitimate actions. Looking slow and far from her best, Serena succumbed to fellow American Sofia Kenin 6-2, 7-5 for her earliest exit at a Slam in nearly five years. "I think I have an fantastic opportunity to go out there and try and play my brand of tennis again, and take it to her as much as I can".
That combined with her performances at the US Open - where she defeated Williams in a controversial contest - and Australian Open suggested that another comeback would be in the cards against Siniakova. I am glad I came.
"I love the crowd, I love playing in big stages".
Naomi Osaka, Serena Williams suffer upset losses
She went 3-1 ahead in the second set before Williams, the 2002, 2013 and 2015 champion, clawed her way back to 3-3. Serena Williams , the 10th seed, was beaten 6-2 7-5 by the 20-year-old and unsung Sofia Kenin .
"Roland Garros brings out my best and I hope it will continue", she added. People tell me, and then I just forget after a second.
"I don't really get it, seriously", Thiem said. "All I know is, she was not ready and it was obvious, I think". Siniakova had only 13 errors in her biggest career win in 1 hour 17 minutes on a sunny and warm afternoon.
I can't stress enough how terrific Kenin played on Saturday.
Unpredictably, this match's results fell secondary as far surprises go, as No. 1-seeded Naomi Osaka's bid to capture a French Open title ended after a crushing defeat to the No. 42-ranked Katerina Siniakova.
Barty plays Kenin in the first match on centre court on Monday (7pm AEDT). Her family moved to NY when she was a baby, and she now is based in Florida.
Osaka, who has yet to reach the second week in four appearances in Paris, got broken three times and failed to take her chances. "For me this is something that I have wanted to do forever, but I think I have to think about it like if it was that easy, everyone would have done it". She ran to her sideline seat and pressed a towel against her face.
In Tuesday's quarterfinals on the other half of the bracket, 2017 U.S. Open champion Sloane Stephens meets No. 26 Johanna Konta of Britain, and No. 31 Petra Martic of Croatia faces 19-year-old Marketa Vondrousova of the Czech Republic.