At 7-6 3-3 and 15-30, Federer looked to have the advantage in the rally against Fucsovics but blasted a routine forehand beyond the baseline.
Federer was then heard screaming something and the umpire quickly issued him with an audible obscenity warning.
After hearing the warning, commentator David Mercer said: “What? Did he use a bad word? I can only assume so.
“That might make worldwide headlines that Federer used a naughty word on court and got warned.”
Federer would then go on to lose the game and in the changeover fellow commentator Chris Bradnam added: “That was very much out of character what we just heard from Federer in that last game.
“Pretty shocked by it actually I have to say. I’m sure many in the crowd are too.”
On Twitter, one fan said: “Roger Federer screaming f*** to a stadium full of people is definitely this week’s mood.”
And Matthew Willis added: “Federer: *shouts ‘F***’ loud enough to disrupt the flight path of a nearby 747.
“Mercer a minute later: “Did Federer use a naughty word.”
After the match, Federer made a slight reference to his frustrations in the match.
Federer said: “A lot matches have not been easy.
“People think when they see the score ‘7-6 6-4 oh yeah that’s straight forward, straight sets’.
“But when you sit in the stadium you really see how close these matches have become at the top level.
“So from that standpoint, I know what it takes and how hard it is and that’s why you never underestimate anybody.
“Sometimes there’s rain delays, sometimes there is waiting for another set with Tsipas finishing.
“All these things play a role in how you come out and play so I was very happy.
“Sometimes it was a bit frustrating. I think that was a lack of rhythm, gusty winds and Fucsovics playing a good match.
“So I really enjoyed myself even though it was tricky. But that’s the challenge and that’s what I’m here for, so I’m happy.”