12:14 PM PT -- Frances Tiafoe just issued a mea culpa for his actions ... saying, "That is not who I am and not how I ever want to treat people."

Tiafoe said he let his frustrations get the best of him ... and he's disappointed with how it all played out.

"That's not acceptable behavior and I want to apologize to the umpire, the tournament, and the fans."

"I'll be better for yall."

Frances Tiafoe absolutely lost it on an umpire on Tuesday ... cussing up a storm as he expressed his frustrations over a controversial call.

The tennis star lashed out following his loss to Roman Safiullin at the Shanghai Masters ... clearly pissed off about how things unfolded earlier in the match.

During the contest, the umpire gave 13th-seed Tiafoe two time violations ... the second one taking away Tiafoe's first serve on a crucial point in the tiebreaker.

After shaking hands with Safiullin, Tiafoe set his sights on the ump ... and let him know exactly what he thought.

"F*** you, man!" Tiafoe yelled while pointing at the umpire. "F*** you!"

"Seriously, man. F*** you! You f***ed me the f***ing match!"

The crowd was in a mix of shock and amusement watching it all play out ... audibly reacting to the meltdown on the broadcast.

Tiafoe -- who made the semifinals of the US Open in September -- should expect a hefty fine for the F-bombs ... as verbal abuse toward umpires is a violation, per the ATP.

Remember, Alexander Zverev was hit with a $40K penalty after calling the umpire a "f****ing idiot" and hitting his chair with his racquet in 2022.

Tiafoe might want to count to 10 next time ... 'cause his outburst will undoubtedly cost him.