Pliskova then hung on to her serve at 3-5 and the watched as Tsurenko made three unforced errors while serving for the match.
It came after Tsurenko rolled her ankle in the second game of the deciding set.
Osaka thrashed the 29-year-old from Ukraine in the quarter-finals at Flushing Meadows last year before going on to claim her first grand slam title, but the second seed was soundly beaten at Pat Rafter Arena.
"I felt like I have control about everything I am doing", said Pliskova, who won here in 2017.
Tsurenko brought up two match points at 5-3, but was held off when her 21-year-old opponent hit three aces and a drop-shot victor to hold.
Pliskova has a 3-2 lead in her overall series against Tsurenko.
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"I feel like in a way that this experience for me is better than winning the tournament", she said.
Much of the attention fell on Osaka, though, who was at a loss to explain why she struggled for fluency against her Ukrainian opponent. "I think today I learned. what I can do to improve the situation".
"I was missing my shots, just not feeling the timing perfectly".
Osaka has never played the Sydney International, where she was expected to headline as she continues her preparation for Melbourne amid expectations of a strong showing after a career year. "I feel more strong now, so I can kind of handle every kind of pressure on court, even when someone like Naomi is playing really strong".
"I'm really happy that finally I've won this title, it's been like seven or eight times at this tournament". "I served well, I returned well - I think that was the key today - and I was moving well, that was fantastic". "I felt like I was too fast on the court today - I felt very good".
Kei Nishikori was playing Jeremy Chardy in the first of the men's semifinals, and Jo-Wilfried Tsonga had a night match against fourth-seeded Daniil Medvedev.