Osaka, chosen to light the Olympic cauldron during the opening ceremony, struggled in an error-strewn display under the centre court roof at Ariake Tennis Park and was knocked out in 68 minutes.
She made a dreadful start by dropping serve in the opening game. The second seed was broken twice more as 2019 French Open runner-up Vondrousova raced away with the first set.
Osaka broke in the second set but relinquished the early advantage with a double fault that allowed Vondrousova to level at two games apiece.
The four-time Grand Slam winner grappled with inconsistency, and even when given a sniff of regaining the initiative she had no response to Vondrousova’s array of crafty drop shots.
Osaka saved two match points as she served to stay alive at 4-5 but Vondrousova converted at the third time of asking as the Japanese star smacked a backhand wide.