Fonseca, at 18 the youngest man in the men's singles draw and on his Wimbledon senior debut, used power and variety in his serve and his trademark brutal forehand to overpower 23-year-old Fearnley, who at 51 is ranked three places above the Brazilian.
Fonseca, knocked out in the first round of qualifying here last year, has raced up the tennis rankings, breaking into the top 100 at the Australian Open and reaching the third round at Roland Garros before losing to British number one Jack Draper. Less than two years ago, he was playing in junior tournaments and caught the eye after reaching the boys' quarter-finals at Wimbledon and winning the 2023 boys' US Open.
"I've been working a lot," he said in an interview on a sun-drenched Court One. "The key is work, believe and dream."
It was the Brazilian's third victory over his "great friend" Fearnley this year, and he capitalised on some nervous and erratic serving from the Briton, who produced 10 double faults in the match, often at crucial moments.
"At the beginning of the match we were both very nervous," Fonseca said. "There were no rallies - we were nervous."
Under pressure to match the free-hitting Fonseca, Fearnley served a double fault to concede the first set and was simply blown away in the second with the Brazilian whipping winners seemingly at will. The stats showed he finished the match with 29 winners to Fearnley's 16.
The British number two rallied in the third set and forced a tiebreak, but the cool-headed Brazilian teenager pulled out two aces to bring up match point and won the game when the Briton put a return long.