Ostapenko was too strong for Svitolina in last year's Wimbledon round of 16 and it was more of the same on the Miami hardcourts as the Latvian sixth seed advanced 7-6 (7/3) 7-6 (7/5).
She next will face either Venus Williams or Danielle Collins, who meet in an all-American quarter-final later Wednesday.
Ostapenko, 20, went out in the first round in Key Biscayne last year but has enjoyed a breakout year since, winning her first Grand Slam title in Paris and racing from 64 to fifth in the world rankings.
She hit 42 unforced errors in an effort to blow Svitolina off the court but offset that with an impressive 44 winners.
"I knew I had to be aggressive and when I got the chance I really went for it," Ostapenko said. "I came here with no expectations, whether it's the first or final I give it everything and do my best."
Dealing with the pressure of being a young Grand Slam champion is something Ostapenko easily takes in stride.
"Defending my title is going to be very special and I am looking forward to going back to Paris," she added.
"The best thing after such a big win like that is how people treat you after you win and that at some tournaments, you get a bye so it makes things easier.
"That can be a negative though, because if you go out early, you don't play many games. I am top-10 player now so everyone wants to beat me."
Just like on Tuesday, windy conditions didn't help either player with a total of six breaks of serve.
Yet in both tie-breaks it was Ostapenko who was able to deal with the conditions more effectively to help seal a place in her first Miami Open semi-finals and help boost a season which so far has belied her talent.
Disappointing runs at Shenzhen, the Australian Open, Doha and Indian Wells compounded a difficult start to 2018, although a quarter-final exit at St Petersburg to eventual winner Petra Kvitova was a more positive return.
Now Ostapenko is determined to take the opportunity with both hands.
"I was rushing too much in the tie-breaks. I had to play smarter but my aggression helped,"Ostapenko said.
"The wind wasn't too bad but I am trying to be more consistent. I am not afraid to miss the ball and that helped me win the match."
Svitolina pleased despite loss
For Svitolina, it was a disappointing result considering she has already won three titles so far this season, but the Ukrainian was quick to admit she deserved everything that came her way.
"I was hurting a little bit, but it was on and off all the time," Svitolina said. "She played a great match. She was much, much better than me if you look at the game.
"Definitely though I'm pleased with the performance I have been showing throughout the hardcourt season.
"I won a couple of titles, the big ones, defending Dubai and then winning in Brisbane so that shows I can play in high level and be consistent."