Sixth-seeded Harrison, the last champion of the Memphis Open, beat Young 6-3, 7-6 (4) in his first round match in the New York Open tournament in Uniondale, New York. During a changeover, the two players could be seen having a heated exchange, but it's unclear what was said.
Young later took to Twitter, writing: "I'm shocked and disappointed, Ryan Harrison, to hear you tell me how you really feel about me as a black tennis player in the middle of our NY match. I thought this was supposed to be an inclusive gentleman's sport."
Ryan pushed back, tweeting: "The accusations made by Donald Young tonight following our match are absolutely untrue. I'm extremely disappointed that someone would say this in reaction to a lost tennis match."
He goes on to say that any audio or video of the incident would clear him.
Meanwhile, the Association of Tennis Professionals is investigating. In a statement, a spokesperson said: "The ATP takes any allegations of racial prejudice extremely seriously. A further review of all video and audio recording from the match will take place as this matter is investigated further. No further comment will be made until the completion of the investigation."