That is the view of ITV Sport analyst Sam Smith.
In the final match of the day on Court Philippe-Chatrier, Williams was beaten 6-2 7-5 by fellow American Kenin in a dramatic contest.
Since returning to the tour last year after the birth of her daughter, Williams has struggled for form due to illness and injury.
Williams entered the French Open far from fully fit as she managed a knee problem.
And Smith believes the 37-year-old was unable to play her best against Kenin due to her lack of fitness.
Smith said: “Great respect from Williams, who knows when she is 60 per cent [fit] she can’t beat someone who is putting on this kind of performance.
“She was beaten out there. She was literally beaten by better tennis.
“She didn’t play horribly, she played OK. Flashes of brilliance, but that’s not good enough when Kenin is playing really lights out tennis.
“She barely missed a ball out there.”
After the match, an emotional Kenin said: “It is a lot of emotions, Serena is such a great player and a true champion True respect to her. Playing against Serena you have got to really fight for every point.
“Serena is such a tough opponent. I am so happy with his win. I am so happy my dad (Alexander) is there in the box.”
Despite her defeat, Williams gave credit to Ruud and said she was hopeful of being fully fit for Wimbledon next month.
Williams said: “I think she played really well. She literally played unbelievable. I was pretty far away – I have not been able to be on the court as much as I would have liked to.
“I will be able to put the time in now. I am glad I came. I love the city and the tournament and I really wanted to be here. It has just been a really grueling season for me.
“I hope I have time to go get fit for Wimbledon. I am still working on it. I think it will be enough time. I definitely hope so. I’m definitely feeling super short of matches. I have some time on my hands and maybe I will take a wildcard at a grass-court event.”