MIAMI — Monday was a bad day for the Florida tennis faithful.

The MacGyvered tennis arena inside Hard Rock Stadium, home of the NFL’s Miami Dolphins, was pretty dead all afternoon. Floridians Danielle Collins and Coco Gauff — the former born in St Petersburg, the latter who has lived in Delray Beach since she was seven — lost to Aryna Sabalenka and Magda Linette respectively.

Then evening fell, a Brazilian named Joao Fonseca entered the building, and a bad day turned into a tennis night out for the gods.

The Fonseca fanatics came from as close as downtown and Miami Beach and as far as Rio de Janeiro and Sao Paulo. They chanted “FON-SAY-CAH” as though they had been echoing his name off the seats of cavernous stadiums for a decade. They razzed his opponent, poor Alex de Minaur of Australia, the world No. 11 and one of the ultimate tennis gentlemen. They ignored the mostly hopeless pleas from chair umpire Mohamed Lahyani to exhibit a bit more sportsmanship than they might at a Copa America final.

“Obrigado, Obrigado,” he kept saying.

Sorry, Mohamed. All those folks in canary kits had a wunderkind they needed to will over the line.

The Fonseca phenomenon has been building for a little over a year now, beginning with two qualifying wins at the U.S. Open. In the third round of qualifying, Fonseca faced a New York crowd in full voice for an American, Eliot Spizzirri, and it got the better of him. Then he won the ATP Next Gen Finals in Riyadh, Saudi Arabia. Things got seriously loud at this year’s Australian Open, when Fonseca blew away No. 9 Andrey Rublev in the first round. The court for his second-round loss against Italy’s Lorenzo Sonego looked like an exclusive nightclub — lines out the door and no hope of getting in.

Things went bananas when he won the Argentina Open in Buenos Aires in February. Maybe too bananas, since he lost his next match, at his hometown Rio de Janeiro Open, and admitted he had struggled with nerves.

In Miami, a city sometimes referred to as the capital of Latin America that is home to some 400,000 Brazilians and a tennis tournament that sells itself as a kind of South American grand slam, the Fonseca chorus reached its first crescendo of many. Against de Minaur, alas, it couldn’t quite get their darling where he needed to go.

For the better part of two sets, Fonseca pounded the Australian with his power, but when he looked up at the scoreboard after nearly two hours, they were somehow even at a set apiece.

He went up a break of serve early in the third set, only to give back the advantage and with it the match. Fonseca would lose five straight games as his 18-year-old arm lost just enough oomph for de Minaur to turn the match into a track meet. He turned what had been a Brazilian highlight reel into an Australian grind, and rope-a-doped Fonseca into submission, 5-7, 7-5, 6-3.

De Minaur scribbled “Rio Open” on a television camera lens when all was said and done, in homage to the crowd and the player he’d found a real handful.

“I did very good not to lose my head,” he said after it was over. “I told myself, you know, I’m going to get chances. I’m going to stick around. Got to stick around. Got to get chances. Keep making it physical.”

Fonseca is going to have nights like these. He is the first man born in 2006 to win an ATP tour title and the fourth-youngest man to win one since 2000. He is likely to win many more. But even he knows how much he still has to learn. After losing his second-round match at Indian Wells earlier this month, he zipped across the desert to Phoenix and won the Arizona Tennis Classic, an event on the second-tier ATP Challenger Tour.

Last fall, before he won the ATP Next Gen Finals, a competition for the best players aged under 21, he spoke of the difference between junior tennis and the lower levels of the pro tours. In juniors, he could mostly win on talent alone. When he started playing grown men at the professional level, even in second- and third-tier tournaments on the Challenger and Futures tours, he quickly saw that smarts often won out over his talent.

In an interview earlier this month in California after his second-round loss to eventual champion Jack Draper at Indian Wells, Fonseca explained the difference between players at that level and those on the ATP Tour.

“They take the opportunities,” he said, a statement that foreshadowed Monday’s battle with de Minaur. Fonseca wanted another chance at one of his break points at 4-4 in the second set, when he sliced a backhand instead of trying to hammer it away. He missed a forehand in the third set to go up 3-1. “When the important points come, they are more aggressive, they know how to play. I’m going to improve in this. The difference today? I didn’t play bad, I think was the mentality.”

Brazilians have known about Fonseca’s mentality and physicality for a while now, and they are so here for it.

Claudia Tanaka, who travelled with her parents 4,000 miles from Rio to cheer for Fonseca, said that she has been watching him since he was a boy. She and her mother and father, both “big tennis fans” who are in Miami for the duration of the tournament, were camped out on the plaza in their canary-yellow Brazil soccer team jerseys, their legs blanketed with the national flag.

“We only had day session tickets today, so we’re going to stay and watch right here,” she said.

A few feet away, Vinnie Paco and Eduardo Paco of Sao Paulo were about to head into the stadium. They decided a week ago to snatch up some plane tickets and come to this tournament. “I wouldn’t be here if it wasn’t for Fonseca,” Vinnie said.

Their compatriots and expatriates turned the first five days of the Miami Open into a Fonseca fest.

Learner Tien, the rising American, might have figured he’d have the home advantage in his first-round match against a player who was a rival through juniors and is becoming one at the tour level. No such luck. Instead, he learned that in Miami, national borders mean nothing. Tien reflected on a 6-7(1), 6-3, 6-4 defeat on Instagram by setting the location for his post to Rio de Janeiro. After the win, Fonseca had written “Am I in Brazil?” on the camera lens.

“I knew it was going to be crowded, but I didn’t know it was going to be, like, huge,” he said of his time in Miami in a news conference Monday.

The Fonseca effect hasn’t been all joy.

Tournament organizers caused some mayhem Saturday after they moved his match against Ugo Humbert from the Grandstand to the main stadium at the last minute. Brazilians who had already secured seats in the Grandstand and were watching Jakub Mensik against Draper as a warm-up act had to scramble to pay a surcharge for seats in the nether reaches of the Hard Rock Stadium. But not before they rained down boos and Fonseca chants in equal measure on an unsuspecting Czech and Brit who were just trying to play their own version of this tournament.

James Blake, the tournament director, had anticipated the situation. He knew Fonseca was one of his meal tickets, despite only being the world No. 60. He knew he was going to have to feature his matches, on pain of Brazilian fury. “It’s a good problem to have,” Blake insisted.

Indeed it is, though Fonseca’s competition doesn’t quite see it that way.

“Super-dangerous player, playing with a lot of confidence,” de Minaur said. “Ultimately, it’s always going to be a tough, tough match against him. He’s still young, so he’s only going to get better, and he’s only going to make it even harder for us guys on tour.”