But the 2017 Brisbane champion said she preferred that to being the underdog.
She has reached the semifinals or better at four of her last five tournaments but hasn't added another title. "I have great memories here and always like to start the year here", Pliskova said.
However, serving for the match at 5-4, she tightened up and Pliskova won 13 of the next 14 points to level the match.
Osaka kicked the air at one point and dropped her racket to the court after missing another, before visibly questioning how she could be getting it so wrong when her forehand skewed wide on game point.
"This helpless feeling I have, I think today (Saturday) I learned what I have to do to, not fix it, but what I can do to improve the situation".
"Rennae helped me quite a bit", she said. "So, hopefully, this isn't a recurring thing".
The Russian was unable to push on from there though, losing the third, and therefore the match, 6-2.
"I feel better with my serve and that I am moving well", she said.
He'll play either Jo-Wilfried Tsonga or fourth-seeded Daniil Medvedev in Sunday's final, where he's hoping to claim his first title since Memphis in 2016.
The Czech Republic's Karolina Pliskova on Sunday secured her 12th WTA title with a victory over Ukraine's Lesia Tsurenko at the final of Brisbane International.
Osaka went into the match as firm favourite but looked flat from the outset as Tsurenko raced through the match in just 65 minutes.
But Nishikori, who had lost nine consecutive ATP finals over 52 tournaments in almost three years, broke the Russian player's serve on four consecutive points in the final game to clinch the match in just 2 hours, 6 minutes, at Pat Rafter Arena.
The two-time grand slam champion showed promising signs ahead of the first grand slam of 2019 with an assured display of baseline hitting against her Spanish compatriot.