The Japanese, who played the first of her four finals here in 2016, could have become the youngest champion here since 2010.
It was the Japanese player who appeared more tired, however, and Osaka, who had not dropped a set during the tournament, hit a series of unforced errors and showed none of the poise and power that earned her a first Grand Slam title.
The Czech was soon a set to the good after unleashing another brutal drive, Osaka caught flat-footed and dumping it into the net.
The 20-year-old buried her head in a towel during the changeover and things barely improved in the second set, Osaka whacking her racquet against her sneakers in anger at her wastefulness.
"I haven't really had much time to rest, so I feel exhausted right now", said Osaka in Tokyo, confessing that she needed a break having won 10 matches in a row including her triumph at the US Open title before her streak came to a shuddering halt.
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Torreira was handed his full debut against the Ukrainian minnows after making some impressive cameo appearances off the bench during the opening weeks of the Premier League season following his £26m summer move from Sampdoria.
"Naomi was maybe a little bit exhausted, you could see that".
"I've literally never felt more exhausted in my entire life", said the 20-year-old, who plans to take some time off. But other than that I feel like I've been playing well and I've been trying to set little goals for myself. Within hours of her 6-4, 6-4 defeat in Tokyo, Wuhan Open organisers said on Twitter she withdrew from their tournament, citing a "viral illness".
Former world number one Pliskova, the fourth seed, ended Osaka's 10-match winning streak in just 63 minutes.
Osaka hammered nine aces over the course of the victory and dropped just eight points in eight service games over the course of the match.
"I think tomorrow is a completely different day", said Pliskova, who admitted to feeling exhausted during the second set.
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"I have played Pliskova twice already and so I know she is a great player", said Osaka, who won their last meeting at Indian Wells.
A weary-looking Pliskova dug deep in the decider and the quality of her superior hitting eventually told as she broke Vekic's resolve with a strong start to the third set.
In clinching the title, Pliskova took a big step towards securing her spot at the season-ending WTA Finals in Singapore, the victory putting her just ahead of Kiki Bertens for the eighth and final berth in next month's field.
"I'm just happy that I also have a title as a married woman".
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