"I felt like it would be better for her to take my pace".
Twelve months ago, the last two women standing were both over 30: Elina Vesnina beat Svetlana Kuznetsova in a three-set thriller, to claim by far her biggest career title, and four years after her only other two.
The overjoyed youngster raced to embrace her family after shaking hands with the umpire, and must now be considered one to watch for the remaining three Grand Slams of the season.
Neither Kasatkina nor Osaka have much experience in big matches with the Japanese player's only career final coming in Tokyo in 2016 and her opponent's lone singles title coming in Charleston previous year. She made the quarterfinals in her tournament debut in 1997 and notched her first win over a top-10 player before losing to eventual champion Lindsay Davenport.
"I played in Melbourne against her, so I knew that she can make a good match". She dropped one set in seven matches.
"I just want to be on the central court, prime time", she said, smiling.
Kasatkina, who was a beaten finalist in Dubai last month, was at it again, prevailing after nearly three hours against No.8 seed Williams, who was two points away from victory at 5-4 in the final set.
Kasatkina will have Saturday to recover before facing Osaka for the first time in their fledgling careers on Sunday.
"I think that's it", she told the crowd. The players exchanged breaks at the top of the first set but from that point on it was all her. "I just lost my concentration", the 26-year-old said.
Williams hit a wayward forehand on the first point and then dumped a forehand into the net on the next point, which caused her to slam her racket in frustration.
"You go for it?" he asked her.
"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".
The daughter of a Japanese mother and Haitian father, Osaka was born in Japan before moving at age 3 with her family to the United States, where she is still based, training frequently at the Evert Tennis Academy in Boca Raton, Fla.
"She's bad. She can not get it", said Kasatkina with a laugh about Osaka's tweener technique. Hit with power and spin it's a formidable weapon, with which she was able to hit through Simona Halep, particularly in the second set.
"She's playing really well".