But the exchange set the tone of the day. McIlroy lost his cool towards the end of his morning foursomes win alongside Lowry, swearing at American fans during a heated moment on the 16th fairway.

With some members of the crowd fuelled by all-day drinking, things became more heated in the afternoon fourballs.

The atmosphere simmered over the opening few holes before becoming a tinderbox on the par-five fourth green.

McIlroy backed off a putt for an eagle three after a shout disrupted his concentration. He missed his effort but Lowry holed his chance – and then erupted with a passionate and pointed celebration towards the offender.

When Lowry stuck away another putt for a birdie on the fifth, he let rip with another vocal celebration. Only his dad Brendan, watching from the side, shouted louder.

Things ignited again on the sixth green when McIlroy backed off another putt. “Can’t take it, Rory?'” bawled one New Yorker.

A crowd in a city known for its brash and harsh sports fans clearly felt this was all fair game.

By this stage, the exasperated European pair – as well as vice-captain Eduardo Molinari – were pointing out offenders to the police officers, who had visibly moved in tighter in a bid to calm the mood.

American opponents Justin Thomas and Cameron Young sensibly appealed for calm, but also stoked emotions with wild celebrations when they won holes seven and nine to leave a tense match all square again.

Messages warning fans of a zero tolerance approach to abusive shouting flashed up on huge screens across the course, saying they would “continue to closely monitor fan behaviour and take appropriate action”.

Shortly after Lowry lunged towards the fan on the 10th tee, one of these reminders was booed by the American fans around the 11th green.

The PGA of America, who organise the Ryder Cup, had already beefed up security around all four matches by this point and the sight of police officers trailing the players was a clear warning sign of the threat they faced.