Scoville Jenkins claimed the biggest scalp on day two at the Parker Pen tournament in New Haven, beating experienced Finn Jarkko Nieminen.
The 18-year-old's 2-6 6-3 6-4 success over the world number 55 was his first win on the ATP Tour.
"I was pretty nervous in the first set but I just kept telling myself to take it slowly," he said.
Jenkins, who is ranked 372 in the world, faces fourth seed Tommy Robredo in the second round.
He has a wildcard into next month's US Open and is keen to make more progress in the senior ranks.
Jenkins added: "The biggest thing is that you have to be professional in everything you do.
"I have been speaking with James Blake and Andy Roddick, and they just told me to keep working and hang in there."
Italian Davide Sanguinetti, Igor Andreev of Russia, Victor Hanescu of Romania and Czech Tomas Zib also won through to the second round.