The second seed, who on Tuesday ended the challenge of Australia's Chris O'Connell, will face either defending champion Daniil Medvedev or Nicolas Jarry.
The first set of a match played in humid conditions hinged on a Machac service game at 3-3 , in which the unseeded Czech's forehand missed its target on break point to give Sinner a 4-3 lead.
The Italian wrapped up the set with a hold to love when Machac was unable to put a second serve back in play.
In the second set, Machac's forehand sailed long to give Sinner the break and a 2-1 lead.Â
The Australian Open champion broke again with a backhand winner for 4-1 and closed out the contest with a serve out wide.
Sinner, a finalist in Miami in 2021 and 2023, produced 16 winners to four unforced errors in a stress-free, 91-minute victory.
1⃣0⃣3⃣ MPH WINNER🥶— ATP Tour (@atptour) Blink and you'll miss it 💥@MiamiOpen | #MiamiOpen | @janniksin pic.twitter.com/J3EsYB4bjDMarch 27, 2024
"I didn't exactly know what to expect today," Sinner said.Â
"The first games I was looking a little bit what kind of tactics could work.
"(Machac) has an incredible talent to play this sport. He is super-fast, he's in great shape."
The win was the Italian's 20th of the year and sets up a potential rematch Medvedev in the semis, in what is developing as a title rivalry.
The pair also met in last year's Miami Open final and January's championship match at Melbourne Park.
Sinner has lost just once this year, falling in the semi-finals of the recent Indian Wells tournament to eventual champion Carlos Alcaraz.
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