By Larry Fine

NEW YORK (Reuters) - The U.S. Tennis Association defended their decision to carry on staging the U.S. Open without the benefit of a covered court even as rain on Sunday postponed the men's final for the third year in a row.

The rain forced world number one Rafa Nadal, seeking to become the seventh man to complete a career grand slam, and Serbian third seed Novak Djokovic to wait until Monday to contest the last grand slam final of the year.

Both the Australian Open and Wimbledon championships have retractable roofs to deal with dodgy weather and French Open officials have discussed the possibility.

"It's technically complex and financially challenging," USTA spokesman Chris Widmaier told Reuters. "At a cost of more than $150 million, do you spend that on a roof or continue to fund grassroots tennis programs in this country?"

Widmaier said $100 million had been spent on improvements at the Flushing Meadows complex in the last five years, with the biggest expense being a new indoor training center.

He also said money from the U.S. Open was funneled into player development, grants and scholarships and to improving tennis facilities around the country, which included providing help to resurface 1,100 courts across the United States.

The tournament's second court, Louis Armstrong Stadium, seats 10,500 compared to 23,771 at Arthur Ashe Stadium.

Four-time U.S. Open winner John McEnroe, now a TV commentator at the championships, said last year he had lobbied USTA officials from the start to put a roof over Arthur Ashe Stadium when it was built to replace Louis Armstrong Stadium in 1997 as the tournament's main stage.

"It seemed like a no-brainer but people at the USTA decided they wanted to build the biggest (stadium)," McEnroe said about the Ashe center court. "But at the same price they could've had a slightly smaller stadium and had a roof."

(Reporting by Larry Fine; Editing by Frank Pingue)