US and Australian Open champion Osaka secured a place in the quarter-finals against Croatia's Donna Vekic.
Osaka said that winning was not everything in Stuttgart, where she last played as a qualifier in 2017.
The reigning Australian and US Open champion has enjoyed a dazzling rise to the top of women's tennis in the last 12 months, becoming the first Japanese woman to win a Grand Slam.
The Serb is almost 2,500 points ahead of Rafael Nadal while Osaka maintains a slender lead over Simona Halep.
Six-time men's champion Novak Djokovic won his opening match under the lights, beating Bernard Tomic 7-6 (2), 6-2.
Osaka lost to Belinda Bencic 6-3, 6-1 in just over an hour in the fourth round of the first title defense of her career.
Venus Williams upset Petra Kvitova in three sets while Osaka beat Mladenovic in straight sets.
With his win, Djokovic went level with Usain Bolt and is now just one behind Roger Federer, who has won it a record five times.
Osaka, who became the first Asian to hold the top ranking after beating Czech Petra Kvitova in the Melbourne final, had worked with the German for just over a year.
Osaka held three match points at 5-3 in the second set but couldn't close it out. She allowed Kvitova to come back and win 23 of 27 points to take that set and go up 1-0 in the third.
Pliskova survived four match points and bounced back from 1-5 down in the third set to defeat Williams for a spot in the final four.
Osaka is bidding to become the first woman since Serena Williams in 2015 to win the US and Australian Opens back to back.
Osaka is the first Japanese woman in the final four at Melbourne Park since 1994.
Australian Open: Naomi Osaka sets up Elina Svitolina last-eight test, Serena Williams faces showdown
The victory left Osaka with a last-eight clash against ambitious Ukrainian Svitolina, who battled past American Madison Keys 6-2, 1-6, 6-1.
Australian Open: Serena Williams relentlessly marches on as Naomi Osaka, Elina Svitolina survive scares
Serena Williams, seeded 16, has dropped just nine games in her three matches so far and with face either sister Venus or world number one Simona Halep for a berth in the quarter-finals.