SHANGHAI (AP) – U.S. Open semifinalist Frances Tiafoe cursed repeatedly at a chair umpire after losing a match at the Shanghai Masters on Tuesday.

Tiafoe sent about 10 expletives in the direction of official Jimmy Pinoargote during a minute-long tirade after exiting 5-7, 7-5, 7-6 (5) against 61st-ranked Roman Safiullin in the third round.

Later, Tiafoe posted an apology on social media, calling his reaction “not acceptable behavior” and writing: “That is not who I am and not how I ever want to treat people. I let my frustration in the heat of the moment get the best of me and I´m extremely disappointed with how I handled the situation.”

The ATP rule book calls for fines of $60,000 at Masters 1000 tournaments for verbal abuse of an official. The tournament can take away prize money, and the case could also be sent to the ATP Member Fines Committee for further investigation.

A tour spokesman said no decision has yet been made on the matter.

The 17th-ranked Tiafoe was angered by being docked his first serve for a time violation at 5-5 in the decisive tiebreaker.

Pinoargote ruled that Tiafoe was not making a genuine attempt to serve when he tossed the ball in the air at the baseline as the serve clock was set to expire. Tiafoe initially protested before continuing to play; he lost that point and the next to close the match.

Tiafoe, a 26-year-old from Maryland, congratulated Safiullin at the net before turning toward the umpire and shouting the first expletives.

Tiafoe passed to the other side of the net toward his chair and said the umpire messed up the match. Peppering his words with more swearing, the American said he had been playing on court for three hours.

Safiullin advanced to face Novak Djokovic.

Tiafoe had a stellar run this summer, reaching the final of the Cincinnati Open, where he lost to No. 1 Jannick Sinner, then reaching the semifinals at the U.S. Open before losing in five sets to fellow American Taylor Fritz.