Sinner downs Michelsen to reach Miami Open quarter-finals
Italian second seed Sinner, who is bidding to follow up his triumph at Indian Wells with victory in Miami, bided his time before completing a 7-5, 7-6 (7/4) win in 1hr 41mins.
The four-time Grand Slam champion, who will face either 19th seed Frances Tiafoe or France's Terence Atmane in the last eight, was made to work hard by the 40th-ranked Michelsen.
After a cagey opening Sinner began to put pressure on Michelsen's serve in the ninth game of the first set, carving out three break points.
Michelsen rode out that storm, but Sinner finally broke through in the 11th game before serving out for the first set.
Michelsen hit back in the second set, breaking Sinner for a 4-2 lead before holding to go 5-2 up.
Michelsen though appeared to struggle with the bright late afternoon sunlight on the Hard Rock Stadium's main court, and Sinner pounced on the reprieve.
Sinner came roaring back to force a tiebreak before sealing the win with a thumping serve that Michelsen could only return wide.
"I feel like I served very well in important moments and that helped me out, especially in the tough moments," said Sinner, who hammered 15 aces to Michelsen's three.
"But today was not easy -- I played a night match yesterday and today in the daytime, so the conditions were very different."
In other fourth round games on Tuesday, Spanish qualifier Martin Landaluce, ranked 151st in the world, upset American 32nd seed Sebastian Korda to score a 3-6, 7-6 (8/6), 6-4 victory.
The win marked another impressive victory for the 20-year-old Landaluce, who eliminated Russian 14th seed Karen Khachanov in the previous round.
Korda, who had beaten world number one Carlos Alcaraz in the previous round, had a match point late in the second set, but Landaluce survived to set up a quarter-final meeting with Czech 21st seed Jiri Lehecka.
Landaluce, who had failed to register a single win at tour level in 2026 before arriving in Miami, dedicated his latest success to his late grandmother.
"She would have been 101 last week, and she passed away a few months ago. I wanted to give her the victory," Landaluce said.
"I'm playing really good this week. I knew I had to do it because there are unbelievable players here. I'm super happy to be here playing with these amazing players."
Lehecka booked his place in the quarter-finals after ousting sixth seed Taylor Fritz in three sets, winning 6-4, 6-7 (4/7), 6-2 in 2hr 25min.
Lehecka credited his "aggressive" approach to the decisive third set for the win.
"I just felt that in the third set, if I wanted to beat a guy like Taylor, I just needed to go for it and be aggressive," Lehecka said.
"I needed to show the courage that I wanted to win the point. That's what I tried to do, and it worked well."
American 22nd seed Tommy Paul set up a quarter-final against France's Arthur Fils after cruising past Argentina's Tomas Etcheverry 6-1, 6-3.
Fils punched his ticket to the last eight with a 6-4, 6-7 (4/7), 6-4 win over Monaco's Valentin Vacherot.