According to The Telegraph, Gimelstob, 41, allegedly attacked and beat a male friend of his estranged wife, Cary. Gimelstob will appear in court on Dec. 12 in Los Angeles to answer charges that he hit venture capitalist Randall Kaplan in the head and face 50 times, shouting, "I'm going to f---ing kill you."
"Of course Justin is very close to me, as a friend and as a coach," Isner said Tuesday on a conference call. "But even if he wasn't, at the current time I would still support him because he's innocent until proven guilty. We'll know the facts soon enough. It might not be a popular opinion, but as of now, I have to give Justin benefit of doubt."
The assault is one of a number of alleged incidents first reported by The Telegraph on Nov. 30. Gimelstob, 41, has taken a leave of absence from the Tennis Channel. For now, he remains on the six-member ATP board.
The 10-member ATP player council, led by Novak Djokovic, will discuss Gimelstob's future on the board this week.
"I don't think [Justin] should take a leave right now," Isner said, "We don't know the facts yet. Hewitt doesn't know the facts, either. He's the guy who was involved in his own nasty situation with James Blake [at the 2001 US Open], and I'm not calling for his head, or for him to step down from working with Tennis Australia. He's being arrogant."
According to documents examined by The Telegraph, Gimelstob allegedly also had an altercation with Kris Thabit, who is a male friend of Cary Gimelstob. The document stated that Thabit had a cut lip and that restraining orders were requested against both men.
In February 2016, Cary Gimelstob sought a restraining order against her estranged husband. She claimed, among other things, that he assaulted her and regularly broke into her home after they separated.
In a statement issued to Metro.co.uk, Justin Gimelstob's lawyer, Shawn Holley, wrote: "Mr. Gimelstob unequivocally and absolutely denies ever engaging in domestic violence or homophobic behavior of any kind. Any suggestions to the contrary are false."
Gimelstob is out on $50,000 bail until his hearing on Dec. 12.