Kouame became the first player born in 2009 or later to win an ATP Tour match with a fighting 5-7 6-4 6-4 victory over American qualifier Zachary Svajda.
Competing as a wildcard in his first main-draw ATP Masters 1000 match, the 17-year-old Frenchman delivered a composed performance to etch his name into the history books.
With his victory, Kouame also became the the youngest Miami match winner and the youngest to claim an ATP Masters 1000 victory since the great Rafael Nadal in Hamburg in 2003.
"It's huge," Kouame told the Tennis Channel.
"First main draw at a Masters 1000 here in Miami, it's such a cool place. I'm really happy to win that match today and hopefully I can win many more.
"It's my first time, so I didn't expect all of this, but here I am. I need to get used to it pretty fast. It brings me a lot of confidence. It shows me that I'm on the right path. I just need to keep working."
At a career-high world No.385, Kouame is the youngest player inside the top 900 and has soared to fourth in the Next Gen Race.
Next up in Miami is Czech 21st seed Jiri Lehecka, as he looks to extend his dream debut.
But while Kouame marches on, Popyrin's slow start to the year continued with a 6-4 6-4 loss to revitalised three-time grand slam finalist and 2014 US Open champion Marin Cilic.
After cracking the world's top 20 last year, Popyrin - the only Australian since Lleyton Hewitt more than two decades ago to conquer Novak Djokovic - has only won two matches thus far in 2026.
Duckworth also lost to a veteran, bowing out 7-6 (7-3) 7-6 (7-4) to Spaniard  Roberto Bautista Agut.
In other matches, former French and Australian Open runner-up Stefanos Tsitsipas beat French-born British qualifier Arthur Fery 6-1 7-6 (7-4) to set up a second-round showdown with Australia's world No.6 Alex de Minaur.
World No.3 Iga Swiatek was a shock first-round loser in the women's event, the five-time grand slam champ falling 1-6 7-5 6-3 to Polish compatriot Magna Linette.
Talia Gibson continued her hot run with a 6-0 6-1 rout of Sara Bejlek while fellow rising Australian Emerson Jones also beat a 20-year-old Czech, Linda Fruhvirtova, 3-6 7-6 (7-3) 7-6).
But fellow Australian Kim Birrell lost 6-2 2-6 6-3 to American Ann Li.
With agencies
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