She lost just eight points on serve against Giorgi to book a meeting with Czech fourth seed Pliskova, who was made to work much harder before ousting Croatian Donna Vekic 6-2 4-6 6-3 in a little over two hours.
"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.
Osaka's defeat was her second in the tournament final, having also been runner-up in 2016, as the 20-year-old seemed to finally succumb to the after-effects of her emotional U.S. Open triumph earlier this month.
The frustration boiled over when Osaka double faulted in the ninth game of the second set to set up Pliskova's second break.
Undaunted by the partisan crowd, the 26-year-old Pliskova was willing to mix it up with the hard-hitting home favorite from the outset, riding her own powerful serve and ground strokes to victory.
The Dutch second seed rallied after losing the opening two games of the decisive set, eventually winning 7-6 (2) 4-6 6-2 in two hours and 22 minutes. "Naomi was maybe a little bit exhausted, you could see that". "I haven't really had much time to rest, so I feel exhausted right now".
"Naomi was playing so good the whole tournament, the future is bright".
The BNP Paribas WTA Finals Singapore presented by SC Global will be played at the Singapore Indoor Stadium from Oct 21 to 28.
The former world No.1, who spent 419 minutes on court en route to the final, celebrated her 11th title in 21 finals.
The players had split their previous two meetings, but Osaka started the final as favourite after her historic NY triumph.
But Pliskova slowly turned the screw and broke for 3-2 with a fizzing backhand her opponent swished well wide.
"I didn't want to try to overpower her because she just has more power than me", said Pliskova.
"I was just happy it didn't go to a third set", Pliskova said.
Fittingly, Pliskova wrapped up the match in the same way Osaka had started it - with an ace - to take victory. Osaka whacked her racket against her sneakers in anger at her wastefulness and, as the strain began to show, she called for her coach Sascha Bajin before groaning: "I feel so stressed out".
"The chances of me playing in the next tournament are very low".