INDIAN WELLS, CA, USA - Two 20-year-olds will contest the first Premier Mandatory final of the 2018 season on Sunday, after unseeded Naomi Osaka followed No.20 seed Daria Kasatkina into the final of the BNP Paribas Open with a 6-3, 6-0 victory over World No.1 Simona Halep on Friday night.

In her third career match against a reigning World No.1, and second against the Romanian while she's occupied the top spot this year, Osaka scored her fourth career victory over a Top 10 player and second this week. 

The World No.44 enjoyed a contrasting passage from Kasatkina, who toiled for nearly three hours and came from a set down to oust No.8 seed Venus Williams, into the biggest final of her career with a statement 63-minute victory.

She flipped the script of the fourth-round match between the two from the Australian Open, which Halep won in straight sets.

"I was kind of nervous that entire game because it kept going back and forth," Osaka said after her win, speaking on the last game of the match. "I'm really glad I was able to finish on my serve instead of having to break her or something like that. 

"I just really tried to be consistent. I think in Australia I just made way too many mistakes, and I sort of handed her the match, so I just tried to be a little bit annoying and return a lot."

From 3-3 in the first set, Osaka won the last nine games of the match to earn her first win over Halep in four career meetings, as she hit 16 winners, broke serve five times and won 67 percent of the points behind her first serve.

As it happened: Osaka rolls past Halep for first No.1 win

She was made to work for her spot in the championship match, though, as she saved four break points in the final game of the match in a game that went to six deuces, but clinched victory on her third match point as Halep buried a backhand in the net.

"I didn't want to look too much on the other side of the court, because I felt then I would lose my focus," added Osaka on Halep's performance.

"I was a little bit surprised, especially the start of the second set, because I feel like she gave points, but I didn't try to think too much about it."

Though Halep also recorded 16 winners in 15 games in the match, she racked up the unforced errors in a total of 27 for the match. 

Halep was also less effective on the seven break points she created in the match, converting on just one, in contrast to Osaka who won five of seven against the Romanian's serve. 

"I didn't feel the ball at all. I couldn't concentrate. I was out of the game today," said Halep after the match. "Osaka is a strong player and she's playing really well. I played in Melbourne against her, so I knew that she can make a good match. She was better. She was more prepared. She was ready to play and ready to win the match. I was not.

"The big positive is that I played semis after so many weeks off, after a tough injury, so I'm not going to destroy this tournament just for this match."

By landing only 56 percent of her first serves overall, Halep was forced on the back foot by aggressive returns from Osaka's side of the court for the duration of the match. 

The 20-year-old won 13 of the 18 points played on the three-time Grand Slam finalist's second delivery in the match, and won 56 percent of points in her return games overall.

The Japanese No.1 will play for a trophy for the first time in nearly 18 months when she takes the court against Kasatkina: her first career final came at Premier-level in the fall of 2016, where she fell to Caroline Wozniacki on home soil at the Toray Pan Pacific Open. 

Osaka and Kasatkina have never played in a competitive match, but they did have a fateful meeting - on the practice court - before this tournament began.

"I feel like it's going to be really interesting, because she kind of plays a completely different game than me," the World No.44 assessed. "I think we're both in the final of this type of tournament for the first time, so it's going to be fun.

"I feel like every player I have played has had the pressure on them because -- like, everyone has been seeded and stuff. I'm just trying to be consistent and not worry too much on winning or losing."