Tiafoe mounted an incredible comeback, from 3-6, 2-5, down to win 3-6, 7-5, 6-2 in the semifinal of the 2021 Vienna Open. But it was a contentious moment while Sinner served for the match at 6-3, 5-3 in the second set, which ultimately proved to be the turning point.
In one of Sinner's serves in the ninth game, Tiafoe could be seen uttering some words, audible enough to cause a hindrance just as the Italian was ready to toss the ball. Tennis has always been a sport of fine margins, and subtle moments like shouting before serves can easily impact a player's concentration.
Sinner conspicuously didn't recover as the incident had a lasting effect on his performance in the match and the rest of his season. Tiafoe broke his serve and went on to level the match.
Sinner would only win two more games, as the American completed the comeback with a dominant third set. It was after the match when Sinner's anger got the better of him, slamming Tiafoe for being disrespectful. In an interview translated from Italian, he said at the time:
"In my opinion Tiafoe, he exaggerated today. He did too much. One thing is when you do the show, the other is when there is no more respect. I don’t know what happened, but today I think it went a little further."
In the same press conference, Sinner stated that he was a big fan of the American's game, but felt "bad" with how proceedings unfolded. Tiafoe brushed off the criticism, saying he was only playing by the crowd's reaction, adding that he "didn't really feel bad" with Sinner's accusation of disrespect.
That match derailed Sinner's momentum. He went on to lose Carlos Alcaraz at the Paris Masters and Andy Murray in Stockholm, both in the second round, and lost ground in the ATP Finals qualification. Sinner missed out on the qualifying proper but did get an opportunity as an alternate.
On Friday, the 22-year-old will look to erase the scar from two years ago when he meets Tiafoe again on the same court. But this time, he'll be looking for a different outcome.