Matija Pecotic should perhaps consider giving up his day job after the full-time finance professional stunned former world number eight Jack Sock on his ATP Tour main draw debut at the Delray Beach Open on Tuesday.

The 33-year-old Croatian qualifier, who is a director of Capital Markets for Wexford Real Estate Investors - an affiliate of the $4 billion investment firm Wexford Capital - rallied to beat Sock 4-6 6-2 6-2.

Pecotic reached a career-high ranking of number 206 in 2015 but temporarily quit the sport due to complications following stomach surgery. He then went on to join Harvard Business School, where he rediscovered his love for the game.

Pecotic sealed the win in front of his boss, who is a long-time friend of his coach Dominik Hrbaty.

"I had to leave work early today," joked Pecotic. "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 number 784, who meets Marcos Giron next, said the win over Sock in the opening round was unexpected but he did not approach the match thinking he did not have a chance.

"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."