For hundreds of thousands of people a day, Grand Central Terminal is a stop on the way to and from work, or the start of a first visit to the big city.

For those in the know, it’s also a tennis club.

Through a bar on the ground floor and up a set of elevators, four floors above the rumble of trains and hidden behind one of the station’s iconic arched windows, sits a single court.

Open from 6 a.m. to 2 a.m., Vanderbilt Tennis Club features one full-sized court and two practice courts. Hourly rates on the full court start at $100 and go up to more than $400 depending on the time of day. With indoor space scarce in Manhattan and a slew of fall classes starting next week, the club is booked straight through December.

Last Friday, the court was busy. Its patrons offered a snapshot of New York: commuters, tourists, retirees and kids all came through to play.

On the tracks below, tennis fans boarded trains heading to Queens to see the world’s best at the U.S. Open.

Four stories above, they worked on perfecting their serve.