Swiatek stunned at Miami Open by 50th-ranked Linette
Reigning Wimbledon champion Swiatek, who won the Miami Open in 2022, suffered her first opening loss at a tournament since the 2021 WTA Finals.
It's just the latest disappointment for Swiatek, who was eliminated in the quarter-finals at the Australian Open and in the WTA 1000 events in Doha and Indian Wells.
Although she was seeded second in the elite combined ATP and WTA hardcourt tournament in Miami, Swiatek dropped this week to third in the world as Indian Wells finalist Elena Rybakina moved up behind top-ranked Aryna Sabalenka.
Swiatek's compatriot Linette had to dig deep after Swiatek -- who won their last encounter 6-1, 6-1 -- raced through the opening set in 33 minutes.
But she stuck to her guns and Swiatek double-faulted to give Linette two break points in the 12th game of the second set, Linette forcing a third set with a return winner.
"I just had to go for a little bit more," Linette told Tennis Channel. "I had to start hitting a little bit faster, too, to push her a little bit more back, not give her space.
"But I think it all started with serving a bit better," she added.
Swiatek didn't go meekly, saving a pair of match points on her own serve in the eighth game of the third and two more as Linette served for the match, before Linette sealed the win as Swiatek sent a backhand wide.
"She actually gets really good on those break points down, match points down," Linette said. "She starts swinging. I just knew I had to stay strong on my serve."
Linette advanced to a third-round matchup against 20-year-old Alexandra Eala of the Philippines, a 6-7 (6/8), 6-3, 6-3 winner over Germany's Laura Siegemund.
"It really was close," said Eala, a surprise semi-finalist last year who needed three hours and 20 minutes to close out the victory.
"It was demanding, physically and mentally. But that was one of my goals last year after the tournament, to improve physically, and here I am fresh as a flower."
It was the longest WTA career match for 29th-ranked Eala, who beat Swiatek in last year's Miami Open quarter-finals for the biggest win of her career before losing to American Jessica Pegula in the semi-finals.
Former world number one Venus Williams, 45, lost her first-round match to Britain's Francesca Jones 7-5, 7-5 after one hour and 51 minutes.
Jones advanced to a second-round match against fifth seed Pegula, last year's Miami runner-up to Sabalenka.
Fonseca books Alcaraz clash
On the men's side 19-year-old Brazilian Joao Fonseca outlasted Hungary's Fabian Marozsan 6-4, 3-6, 6-2, to book a second-round match with world number one Carlos Alcaraz.
Fonseca, ranked 39th, was strongly backed by Brazilian fans as he booked a first-ever meeting with Alcaraz and his second marquee contest in as many tournaments after falling to Jannik Sinner at Indian Wells last week.
French 17-year-old Moise Kouame beat qualifier Zachary Svajda 5-7, 6-4, 6-4 to become the youngest player to claim an ATP Masters 1000 victory since Rafael Nadal in Hamburg in 2003.
Kouame, who received a wild card invitation, next faces 21st seed Jiri Lehecka of Czech Republic.
Kouame, who battled through cramping late in the match, was thrilled to receive a message of congratulations from his idol Novak Djokovic.
"He texted me something really, really nice," Kouame said. "I don't know how I'm going to answer. Hopefully I can meet him... it will be my dream."