Fourth seed Kenin, only 22, was swept aside by Estonian 35-year-old Kanepi 6-3, 6-2, with the young American short on match practice, becoming the earliest reigning champion departure at the Melbourne event since Jennifer Capriati in 2003.
Kenin is also the highest ranked player across both men’s and women’s singles draws in 2021 to be knocked out, joining a long list of seeds to bow out early, admitting the pressure of the situation got to her.
“Obviously I haven’t experienced that,” she said. “I felt like I’m not there 100 per cent physically, mentally everything just feels real off. It’s not good.
“I know I couldn’t really handle the pressure. I’m not used to this, so right now I’ve just got to figure out how to play at that level that I played at.
“Because like today and those matches, it just hasn’t been there.”
Kenin was the only women’s seed to lose on day four, with top seed Ashleigh Barty getting past compatriot Daria Gavrilova 6-1, 7-6 (9-7), despite a heavily strapped quad.
“It’s actually a very small tape job, we just wrap it to make sure it stays on,” Barty said.
“I cant really hide it. I’m fit as a fiddle. I’m good to go.”
Fifth seed Elina Svitolina eased past Coco Gauff in straight sets, along with sixth seed Karolina Pliskova against Danielle Collins.
In the men’s singles, Rafael Nadal’s biggest challenge was a heckler among the crowd as he won 6-1, 6-4, 6-2 over American Michael Mmoh.
A woman was ejected from the Rod Laver Arena for flipping the middle finger at Nadal and calling him an “OCD f***”.
Nadal laughed off the situation, when he said: Maybe she had too much gin or tequila. I don’t know. Honestly it was strange situation, but funny at the same time.”
Fifth seed Stefanos Tsitsipas survived a five-set epic with local hope Thanasi Kokkinakis, while another Aussie, 21st seed Alex de Minaur, got through in straight sets.
Russian pair Daniil Medvedev and Andrey Rublev both progressed to the last 32, along with ninth seed Matteo Berrettini, while seeds Borna Coric and Lorenzo Sonego were knocked out.