There was a shocking incident in the first round of qualifying for the US Open as a fan verbally attacked tennis coach and former world No. 12 Viktor Troicki.

Former Serbian player Troicki now coaches compatriot Hamad Medjedovic, and was watching the 20-year-old play against Croatian player Borna Gojo in the first round of qualifying for the last Grand Slam of the year.

However, Troicki was suddenly interrupted by an aggressive fan who swore at him after he claimed the 37-year-old coach told him to 'shut up' because he cheered for Gojo.

The fan's outburst was caught on video and posted on social media. In the clip, he calls Troicki a 'f**king crybaby', before adding:

"He can’t take someone cheering against his f**king player. It’s the US Open. There’s 12 million f**king people in this city. We all are f**king loud. F**king clown."

Security guards came and stood between the player and Troicki, but the fan's rant didn't stop there. Someone who appeared to be with him tried to pull him away, but he told them to 'f**k off' before continuing his outburst.

"Who’s provoking? We’re cheering, you d**k. It’s New York, not f**king Serbia," he continued.

The other spectators can be heard reacting with shock and dismay to the fan's actions. One woman started shouting that she was going to call 911, the number for the emergency services in the US. Meanwhile, others started jeering at the fan and putting their thumbs down, while one said:

"Come on, that’s a former player, man. You’re talking to Viktor Troicki."

On social media, tennis fans have had a similar reaction.

"Shameful behavior by a “fan” who came to provoke. Probably doesn’t know what tennis is and how fans should behave," wrote one.

"Such a threatening and dangerous behaviour towards the players and coaches should not be tolerated and those aggressive individuals should be removed from the whole complex and issued the ban for the entire tournament!" added another.

Medjedovic's opponent Gojo won the match 7-5, 6-7 (2), 6-4, and will go on to face Adrian Andreev in the 2nd round of qualifying for the US Open.