The pair exchanged words and had to be separated by chair umpire Mohamed Lahyani.
It had been a heated match, with Kokkinakis angry about a few calls, but things escalated when Harrison allegedly sledged the Aussie teenager at the net.
It seemed to be a matter of Harrison taking his personal dislike of Nick Kyrgios out on Kokkinakis, who is well liked and respected on tour.
Tennis journo Ben Rothenberg was courtside for the whole drama.
Kokkinakis-Harrison is getting NASTY. Mo Lahyani just leapt out of his chair to separate the two after the first set. #CincyTennis
— Ben Rothenberg (@BenRothenberg) August 16, 2015Here's the confrontation.
It's kicked off today between Kokkinakis and Americab Harrison... #neverstops pic.twitter.com/iQFzV4m4RA
— Brenton Speed (@BrentonSpeed) August 16, 2015
Backstory: Kokkinakis was enraged by Lahyani TWICE overruling Harrison would-be DFs on break points. Harrison didn't like the arguing…
— Ben Rothenberg (@BenRothenberg) August 16, 2015…clean-cut all-American Harrison started muttering things mocking Kokkinakis (maybe in earshot?) about his "37 tattoos and 23 earrings"…
— Ben Rothenberg (@BenRothenberg) August 16, 2015…then the two argued at changeover. Kokk thought Harrison called him a "prick," RH said he had just said "kid," which TK didn't like either.
— Ben Rothenberg (@BenRothenberg) August 16, 2015"Don't sledge me, mate! Don't sledge me, it's not cool." -Kokkinakis, muttering just now in slang that we all learned this week!
— Ben Rothenberg (@BenRothenberg) August 16, 2015After Mo separated them, Kokkinakis left for bathroom break and Ryan shouted "your whole crew is just so cool!" (presumably re: Kyrgios).
— Ben Rothenberg (@BenRothenberg) August 16, 2015Lahyani overruled AGAIN on break point, this time against Harrison. Stay for the face at the end… pic.twitter.com/rC6gDJebMN
— Ben Rothenberg (@BenRothenberg) August 16, 2015Harrison wins second set 6-3, glares at Kokkinakis as he walks to his seat. #imjusthereforthehandshake
— Ben Rothenberg (@BenRothenberg) August 16, 2015Harrison gets code violation for unsportsmanlike conduct (more cumulative than anything). Kokkinakis: "Should've happened long time ago."
— Ben Rothenberg (@BenRothenberg) August 16, 2015Kokkinakis complaining as Harrison argues a call. pic.twitter.com/2O7WjB04Xw
— Ben Rothenberg (@BenRothenberg) August 16, 2015"He's 19. If he wants to get into it, I will bury him. Wawrinka should've decked Kyrgios, and I should deck that kid." -Ryan Harrison
— Ben Rothenberg (@BenRothenberg) August 16, 2015In general, Harrison seemed to be taking out all of his dislike for Kyrgios (and there's a lot) on Kokkinakis, conflating the two…
— Ben Rothenberg (@BenRothenberg) August 16, 2015