"I wasn't that aggressive, " Osaka said.
Asked how it feels to be in the final at a tournament like Indian Wells, Osaka's response is not what one would expect.
Kasatkina and Osaka have never before met at Tour-level, perhaps unsurprising considering both are just 20.
With Halep being the world No. 1 and Williams being a seven-time Grand Slam champion, these results got the tennis world's attention. Looking poised to make a run to reclaim the title she won in 2015, she instead put up a performance full of uncharacteristic errors and showed little fight.
"I don't really know what's going on right now".
"I feel like there is a new generation", Osaka said, "and we're trying to push through".
Most expected Halep to come out firing in the second set but to everyone's surprise she totally capitulated, losing her serve three times as Osaka raced to victory.
Kasatkina paid the price for being too tentative at the start of the second, Osaka on a mission as she broke in clinical fashion and held to lead 2-0.
"This one I will try to forget as fast as possible", Kerber said.
"I'm really happy I got to the finals, but for me it's not over until I win or, like, the day is over".
Osaka (20 years, 153 days) is the youngest Indian Wells champion since Ana Ivanovic (20 years, 138 days) lifted the trophy in 2008.
This is unprofessional. I wish Simona would just scream or something.
Kasatkina took advantage of a sub-par serving night from the more powerful Williams as the match progressed, forcing long rallies that ultimately wore her 37-year-old opponent down.
After beating Katerina Siniakova in the round of 64, Kasatkina beat 2017 US Open champ Sloane Stephens, 2018 Australian Open champ Caroline Wozniacki, two-time Grand Slam victor Angelique Kerber and Williams, in that order.
Until Indian Wells, Osaka had made it past the quarterfinals at a WTA event just once, when she was runner-up in Tokyo 18 months ago.
"I'm ready, one match to go", she told the crowd in her on-court interview.
Osaka had also beaten Russia's Maria Sharapova, Poland's Agnieszka Radwanska and Czech Republic's Karolina Plíšková on her way to the final. She could move to 15th or 16th, and has a shot at the top 10 if she would win the title.
Coming into the semi-final, Osaka had spent 455 minutes on the court while kasatkina had surprised herself with her lack of three-set grinds having spent just 317 minutes ahead of her match with Venus Williams. It was really tough match, so...