Thanasi Kokkinakis, Harrison in heated ATPCincinnati masters verbal

Thanasi Kokkinakis has been met with threats of violence by American pro Ryan Harrison in a heated on-court clash.

Harrison and Kokkinakis had to be separated by tournament officials after their tight qualifying encounter in Cincinnati.

Tensions between the 19-year-old Greek - Australian and 23-year-old American bubbled over during the match, won by Kokkinakis 7-5 3-6 7-6 (7-2).

Post-match, Harrison told the chair umpire "You have got to get these kids under control or they're gonna get hurt."

While Kokkinakis could be heard telling the tournament referee: "I've done nothing to disrespect him and he's giving me that."

According to witnesses, during the match Harrison shouted: "Your whole crew is just so cool" at Kokkinakis, referencing last week's drama.

New York Times tennis journalist Ben Rothenberg quoted Harrison as threatening violence against the rising South Australian.

"If he wants to get into it, I will bury him. Wawrinka should've decked Kyrgios, and I should deck that kid," Rothenberg quoted Harrison as saying.

Harrison received a warning for unsportsmanlike conduct, prompting Kokkinakis to say: "Should've happened a long time ago," according to Rothenberg.

Kokkinakis  then told Harrison: "Don't sledge me mate, don't sledge me, it's not cool".

Thanasi Kokkinakis will continue his effort to qualify for the prestigious US Open lead-up event against Turkish world No.84 Marsel Ilhan.

In the main draw, Kyrgios was handed a rematch with his Wimbledon conqueror Richard Gasquet, while Bernard Tomic will play a qualifier to set up a potential second-round meeting with world No.4 Kei Nishikori.