Modell’s Sporting Goods is opening a new flagship in the former Sports Authority space at 845 Third Ave. on the northeast corner of East 51st Street.

The Rudin-owed 21-story tower takes up the entire block front between East 51st and 52nd streets.

The 21,000-square-foot store encompasses the ground and second floor along with a storage basement.

Robert Steinman represented Rudin Management in-house. Asking rent for the ground was $250 per square foot.

Last spring, Modell’s had considered and rejected a joint bid with Sports Direct that would have divvied up all the Sports Authority stores prior to its bankruptcy filing. Later during the bankruptcy process, Dick’s Sporting Goods put its dibs on 28 stores, but last month, after due diligence, handed this one back to Rudin.

Knowing his interest, Bill Rudin contacted Modell’s Chief Executive Mitchell Modell, and within two weeks, shook hands on a 10-year lease that was signed on Monday.

“He was very excited to open there,” said Rudin. “Mitch and I have known each other for a long time, and it’s great when business and friendship merge. His is a quintessential New York family and so are we.”

“It will be our flagship store with expanded men’s and women’s assortments,” said Modell.

Including this one, Modell’s has four stores opening in November with two in New Jersey, and one in Brooklyn to encompass 44 in the five boroughs and 159 overall.

Modell predicts the holiday retail season will depend on the weather and the local teams. “If it’s a cold December, and the Giants and Knicks go long into the season, it will be good. But if it’s a warm winter, and the teams are lousy, it remains to be seen. If it is 82 [degrees] like last week, we’ll get out T-shirts and swim trucks.”