The 29-year-old German lost 6-2 6-2 to Russia's Ekaterina Makarova.
Cutting a troubled figure on court a world away from the feisty baseliner who last season battled her way to two grand slam titles, Kerber lacked the pace and power to trouble her fellow left-hander.
Also in seeded women's action, Timea Bacsinszky, the No. 30 seed, bested unseeded Sara Sorribes Tormo in her opening match, 6-1, 6-2.
Makarova nailed three games in a row into the second set, before Kerber broke in the fourth game in an unfamiliar manner. This also marks the second consecutive year in which the German player has been bounced from the tournament in the first round.
Kerber struggled with 25 unforced errors against just 16 winners, and won less than 50 percent of points behind her first serve. In the last game, Kerber held seven break points but Makarova fought off each one despite, she said, "fighting with my emotion".
Bacsinszky reached French Open round two after winning 6-1 and 6-2 over Sorribes in an hour and 22 minutes. She'll be defending final points at Wimbledon, where she could again lose the top ranking, this time to someone other than Serena Williams.
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It was the second successive first round exit at Roland Garros for world No. 1 Kerber, the reigning US Open champion. The 31st-seeded Vinci, most famous for ending Serena Williams' hopes of a calendar year Grand Slam by beating her in the semifinals at Flushing Meadows in 2015, lost to Rio Olympics gold medallist Monica Puig 6-3, 3-6, 6-2.
"I'm happy with the game, of course", Kvitova said, "but I mean, it wasn't really about the game today". I have to find myself again and try to forget the clay season as soon as possible, and reset for the grass season again.
The two-time Wimbledon champion downed outclassed Julia Boserup of the United States 6-3, 6-2, falling to her knees in celebration before weeping at the net.
"At the end of the match, it was really tough as I knew she wouldn't give it to me - I had to win it".
A reporter then told the Czech player that her racket throwing might be a sign that she is already capable of tapping into her competitive instincts.
The two-time Grand Slam victor lost 6-2, 6-2 to 40-ranked Ekaterina Makarova on day one at Roland Garros to become the first major casualty of the tournament.
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