Sabalenka wobbled on serve early in her first match since claiming the Madrid title last week and squandered three chances to break Kenin before finding her groove to level a pulsating opening set at 4-4.
The players traded breaks again to set up a tie-break where Kenin displayed glimpses of the form that marked her 2020 Australian Open title run to grab the early advantage following an unforced error by Sabalenka.
Kenin, who was once ranked number four but has slipped to 134 in the world due to illness and injuries, heaped pressure on the reigning Melbourne Park champion in the second set for a 4-2 lead and held firm for a famous victory.
Earlier, 2013 runner-up Victoria Azarenka (33) snapped the six-match winning streak of Saint Malo champion Sloane Stephens (30) with a 6-4, 6-3 victory over the American.
However, it was the end of the road for another former world number one in Karolina Pliskova (31), who fell to a 7-6, 6-2 defeat by qualifier Anna Bondar (29) while third seed Jessica Pegula (29) lost 6-2, 3-6, 6-3 to fellow American Taylor Townsend (27).
Last year's French Open runner-up Coco Gauff (19) continued her preparations for the year's second Grand Slam by dropping only one game in her victory over Yulia Putintseva (28).
"I usually have long matches with her," Gauff said. "I was fully prepared for that. But I told myself to play on my terms.
"She's a tricky player, has some big wins. Made the quarters of the French a couple times. I know clay is her surface. It's also kind of mine, too, so it was a good match today."
On the men's side, Belgian David Goffin (32) rallied past Luca Nardi (19) 3-6, 6-4, 6-2 to secure a meeting with 2017 champion Alexander Zverev (26) before Madrid quarter-finalist Daniel Altmaier (24) overcame another Italian in Giulio Zeppieri (21) 7-6, 4-6, 6-0.
Marco Cecchinato (30) delighted fans at the Foro Italico with a 6-3, 7-5 victory over Mackenzie McDonald (28) and he plays Roberto Bautista Agut (35) next.
Novak Djokovic (35) faces Tomas Martin Etcheverry (23) on Friday while Carlos Alcaraz (20) will take on Albert Ramos-Vinolas (35) a day later.
