The unseeded Osaka, from Japan will play in her first premier mandatory event final on the WTA Tour against another 20-year-old, Daria Kasatkina of Russian Federation who beat Venus Williams 4-6, 6-4, 7-5.
The 20-year-old Kasatkina rallied to claim the biggest victory of her career and defeat seven-time Grand Slam victor Williams, extending a year-long assault on top players, Grand Slam winners and former world number ones.
Kasatkina will now look ahead to the final and hope she still has some energy after going two hours and 48 minutes against Williams. Things were looking good after she held serve to go ahead 5-4 in the set and dug Kasatkina into a 0-30 hole in the 10th game.
Halep was the overwhelming favourite but the three-time grand slam finalist was outclassed by hard-hitting Osaka 6-3 6-0 on Friday.More news: Sonic to unveil pickle juice slush in 2018
"Anyone who gets used to losses should give up on life", Williams said.
"Maybe one of the best nights in my life, for sure".
"I feel like there is a new generation and we are trying to push through", said Osaka.
Kasatkina is now 2-1 against Williams, having won in Auckland in 2016 and lost at Wimbledon later that year.
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. "I missed the ball a lot and I didn't play what I had to play".More news: PM Modi to scientists: Can you develop more efficient solar modules?
That drop-shot variety earned Kasatkina a quick break in the second set, but again Williams hammered back to level, 3-3, as the Russian's serve buckled with a double fault. Sometimes I was even smiling on the court... you just catch yourself-you're in night session, you're playing against a legend, and you are in the third set. They remained on level peggings to 5-5, but by now the oldest woman in the draw began to look tired as she battled to hold serve.
Osaka sailed past a below-par Halep as the No1 leaked errors to concede nine straight games, but the Romanian will retain the No1 ranking as the tour heads to the next Premier Mandatory in Miami.
For Kasatkina, No.8 seed Venus Williams was the fourth consecutive Grand Slam champion she's beaten in a row. Halep is seeking her second Indian Wells title after beating Jelena Jankovic in 2015.
Kasatkina fired one ace, hit five double faults and broke Williams's serve seven times in the entertaining match.More news: Britain's foreign secretary says it's 'overwhelmingly likely' Putin ordered ex-spy's poisoning
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