Full-time real estate professional Matija Pecotic stunned former world No. 8 Jack Sock on his ATP Tour main draw debut at the Delray Beach Open on Tuesday.
Pecotic, who works as a director at a real estate investment firm, plays tennis part time and beat Sock 4-6, 6-2, 6-2 to set up a tie with American Marcos Giron in the round of 16 on Wednesday in Florida.
The Croatian said he had to "leave work early" to play his match against Sock.
"I had to leave work early today," Pecotic said. "I had to send an email to the whole team. [My boss] let me off. I'm going to have to ask for another day off tomorrow."
The world No. 784 had a career-high ranking of No. 206 in 2015, but his tennis career was dealt a blow when he had complications after surgery. The 33-year-old refound his love for the sport when he went on to study at Harvard Business School.
He downed Sock in two hours and 10 minutes, rattling off 10 aces and 30 winners in the first-round match.
"You've got to be realistic," Pecotic said. "This is a former top-10 guy with an incredible amount of tennis experience and a huge serve. He came out serving 134 mph on the first serve. It would be arrogant to think I'm going to come out and expect to win.
"But I figured, if I could sink my teeth into the match and work on the two or three patterns I prepared before, that I'm going to have a chance."
On his training schedule, Pecotic said he finds "creative ways to work around it."
"I absolutely love this game, and I know it's not forever and I'm 33," he said after qualifying for the main draw.
"I try to maximise each day. I try to train every morning if I can, five, six times a week. Sometimes I train with my boss, who is 70 years old. This week I trained with a guy who is probably in his late 50s. But you find creative ways to work around it."
Giron, Pecotic's next opponent, is ranked No. 55 and beat Australian Open quarterfinalist Ben Shelton in the first round.