"I wasn't that aggressive, " Osaka said.
Williams won her first four matches at Indian Wells in straight sets, including a 6-3, 6-4 victory over sister Serena in the third round. Osaka could find herself being yanked around the court at Kasatkina's will.
Halep's error-riddled performance was contrasted with Osaka's steady and assertive tennis, as well as against the scillintating show Kastatkina and Venus Williams put on display earlier. As much as Kasatkina has joked that her head is empty when she players, she probably has the best tactical mind of her generation. "But I didn't. I was just sort of more relieved that it was over". But I feel like we ended up in the same place, and we're both going to try really hard.
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.
"[After a] match like this, you're just speechless", she said. She was then broken in the following game, and completely lost the plot at the baseline.
"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".
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Kasatkina has not let up at Indian Wells, with wins over US Open champion Sloane Stephens, world No 2 Caroline Wozniacki and world No 10 Angelique Kerber to set up her showdown with Williams. "See you next year, Indian Wells!".
In her post-match interview, Osaka sounded a warning that she and her peers were ready to challenge the established order. "I just knew that she was going to fight for every point, too, so I couldn't afford to lose points based on nerves, and I had to keep making the right decisions". Kasatkina won on her second match point when Williams netted a backhand.
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 over the course of the contest.
Neither Kasatkina nor Osaka have much experience in big matches, with the latter's only career final coming in Tokyo in 2016.
As for Kasatkina, she had picked up her first title while still a teenager, in Charleston a year ago while still ranked 42, and had since gone on to beat the best out there.
"This was my first breakout tournament when I was 16 so to come back and get closer to hopefully a title, it's like a dream come true", said Venus, who was taught the game by her father on the courts of Compton, California.