American flags will be flown at half-mast as New York remembers the 2,977 people who died when hijackers crashed passenger jets into the Twin Towers in Manhattan on that day in 2001.
That year's US Open had concluded just two days earlier, with Lleyton Hewitt winning the men's singles title.
American player James Blake had lost in the second round that year and was in Augusta, Georgia, on Tuesday, 11 September.
But the New York-born future world number four says the day "shattered the guise of safety" felt by Americans and many professional tennis players.
"It was something that I just couldn't believe could happen in the States," Blake told BBC Sport.
"I don't know if we were all naive, or living under the guise of safety and feeling that everything was going to be OK, but it kind of shattered that for a while.
"There was a lot of panic, at every level - whether it be from the highest of highs in the political world to the people on the ground, to everyone around Ground Zero. "
His father, though, was still in New York, where he worked. With mobile phone networks overloaded, there was an uncomfortable wait to discover he was safe and well.
Blake was living in Florida at the time, and with planes grounded, took a hire car from Augusta back to Tampa.
His next tournament was in Hong Kong two weeks later, and he admits it was far from easy to board the plane.
"It was definitely scary because I think you get so isolated in tennis," he said.
"You feel like the little bubble you are in, where you are a kind of travelling circus, is normal.
"And then when you realise how restricted travel can be, it wakes you up a little bit to the reality and how quickly things can change.
"It felt like there was a safety net around you. We lived in a bit of a dream world where you are living your dream of playing a sport for a living, and that can all be taken away. "
Many domestic sporting events were paused in the aftermath of the attacks, and the Ryder Cup - which had been due to take place at The Belfry in England later that month - was postponed by twelve months.
In Blake's view, the return of competitive sport helped enormously.
"I think that was one of the things that brought the country back," he said.
"Sports were stopped, but when they came back having [then President] George W Bush throwing out the first pitch at a Yankees game - I'm not a Yankees fan, and I can't say I was necessarily a George W Bush fan - I was almost in tears at that moment.
"It was special to have a sitting president [do that]. And it also signified we are back - we are going to have sports, we are going to continue on with our lives.
"The world seemed like it was back on track in a small way. "
Before the night session - and the all-American semi-final between Ben Shelton and Frances Tiafoe - a giant flag will be unfurled on court as R&B artist JoJo sings The Star-Spangled Banner.
"To me it's a really poignant moment," Blake added.
"The 2001 US Open, I remember so well. What would have happened if it happened two or three days earlier, and we were all still there?
"To be perfectly honest, a lot of times I feel playing the national anthem at smaller sporting events is unnecessary.
"But for the US Open, being such an international event and having it around 9/11, I I think it is a great moment.
"It gives you a chance to reflect. "
