The hard-serving Czech spoiled the party in front of a decidedly pro-Osaka crowd in Tokyo on Sunday, taking the Pan Pacific Open title with a 6-4, 6-4 win over her Japanese opponent.
The players had split their previous two meetings, but Osaka started the final as favourite after her historic NY triumph.
Pliskova took advantage, capitalising both times when presented with a break point and winning the last 11 points to secure her second WTA title of the campaign.
Pliskova had battled through three three-setters to reach the final this week, saving two match points against Alison Riske in the quarterfinals, in contrast to Osaka's smoother passage through her half of the draw.
"First of all my serve was very good today - I was able to hold all my service games, so that was the key", said Pliskova, who needed just 63 minutes to dispatch Osaka. "Naomi was playing so good all tournament but I think she was a little bit exhausted today".
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Osaka and Pliskova will face off Sunday for the third time overall and second this year with a Premier-level title on the line.
The 20-year-old, who became Japan's first Grand Slam singles champion in NY two weeks ago, hit a further eight aces to overwhelm world number 37 Giorgi.
Osaka, the runner-up in Tokyo two years ago, climbed to seventh in the rankings after becoming Japan's first grand slam singles champion but she looked lost for ideas against Pliskova.
As the strain began to show, a dejected Osaka called for coach Sascha Bajin and groaned: "I feel so stressed out!". "But the future is bright for her and congrats to her for an incredible few weeks", said Pliskova, 26, after her 11th career title.
She could barely bring herself to speak at the trophy presentation. "She was playing really well, and my serve helped me". Pliskova said after the match.
But Osaka was still proud she had made the Tokyo final and added: "To play the final of the US Open and then to come here, I think I did pretty well".