By Larry Fine

NEW YORK (Reuters) - Unrelenting rain washed out play at the U.S. Open on Friday, sending the tournament to a Monday finish for the second year in a row.

The final grand slam of the year had been treated to near-perfect weather for the first 10 days but heavy rain over the last 24 hours wreaked havoc with the schedule.

"Certainly today is a damn shame," four-times champion and TV commentator John McEnroe told Reuters about the lost day at Flushing Meadows.

Thursday's rain-suspended quarter-final between world number three Rafael Nadal and Chile's Fernando Gonzalez will now open Saturday's program on Arthur Ashe Stadium to see who will face Argentine Juan Martin del Potro in the final four.

The Nadal-Gonzalez match will be followed by the men's doubles championship and the women's semi-final between Caroline Wozniacki of Denmark and Belgian Yanina Wickmayer.

However, officials said the doubles final and the Wozniacki-Wickmayer semi-final could be moved out of the stadium if earlier matches run long.

The highly anticipated women's semi-final between number two seed Serena Williams and 2005 champion Kim Clijsters will now be a prime-time affair, concluding play Saturday night.

The so-called Super Saturday scheduling of men's semi-finals along with the women's final on the day before the tournament's Sunday finish has always made bad weather toward the end of the Open fortnight a dicey proposition.

"Unfortunately it's always the roll of the dice when you get that Thursday night match," McEnroe said about the last quarter-final, as Nadal and Gonzalez can attest.

"It can be a big problem," he said about a delay that carries the match over to Friday. "The winner doesn't have any leeway and has to play three in a row.

"Now we're talking about a Monday final. There's not a lot of margin for error. It also makes it tougher for Rafa (Nadal) to win it."

NADAL LEADING

When rain halted the last men's quarter-final Thursday night, third-seeded Nadal led number 11 Gonzalez 7-6 6-6 with the Chilean serving at 2-3 in the tiebreak.

The 23-year-old Nadal, hoping to win the one major title to elude him, was suffering from a strained stomach muscle and could be aided by an extra day of rest.

But in order to realize his dream, the Spanish left-hander would still have to play three matches in three days, with the men's semi-finals on Sunday and the men's final on Monday.

The other side of the men's draw has already set up a semi-final between five-times champion Roger Federer and 2007 finalist Novak Djokovic, who played their quarters Wednesday.

Rain also forced last year's U.S. Open to conclude Monday, the first time since 1987 that Mother Nature had added an extra day to the hardcourt major.

With two straight years of bad luck, the debate about constructing a roof over 23,000-seat Ashe Stadium will likely be renewed.

Both the Australian Open and Wimbledon have roofs to deal with inclement weather.

"It seemed like a no-brainer but people at the USTA (U.S. Tennis Association) decided they wanted to build the biggest (stadium)," said McEnroe.

"But at the same price they could've had a slightly smaller stadium and had a roof."

(Editing by Steve Ginsburg)