When it comes to raising the roof at the US Open to guard against rainy days such as Friday, the reigning singles champions on the Flushing Meadows hardcourts are divided.

Roger Federer favors the security of knowing a schedule can be played as planned while Serena Williams just thinks it would be really strange to put a lid on 23,000-seat Arthur Ashe Stadium to provide cover from the rain.

Just last year, Federer was forced into a Monday men's final, beating Britain's Andy Murray for a fifth consecutive US Open crown after Tropical Storm Hanna halted a Saturday semifinal between Murray and Rafael Nadal.

Federer sees a roof as practical if not vital.

"Is it necessary? Maybe not. I don't know," Federer said as the fortnight began. "With a roof you can make it more predictable for fans, sponsors, TV, players. That's why it's a good thing to have. That's why I'm obviously for it."

But Williams, whose women's semifinal against Belgium's Kim Clijsters was delayed Friday by rain, was concerned the unique atmosphere of Ashe Stadium might be compromised by playing under a covering.

"It would be hard to add a roof on this wonderful stadium so I think we're fine," she said. "Historically we've done well. It would just be kind of weird to put a roof on Arthur Ashe Stadium."

Showers began Thursday night, halting a quarterfinal between Spanish third seed Nadal and Chilean 11th seed Fernando Gonzalez. Nadal, hoping to complete a career Grand Slam with his first US Open crown, was ahead 7-6 (7/4), 6-6 (3/2).

Forecasters called for a 100 percent chance of rain to Saturday mid-day and 70 percent until nearly midnight, all-but ensuring schedule disruptions.

Drying from machines and towel crews would be unable to begin until the downpour concludes.

The Australian Open has protection from the elements and Wimbledon's retractable roof debuted this year. French Open officials have plans, but no guarantee, for a roof within five years.

That could leave the US Open one day as the lone holdout exposed to the elements, something that surprises world number one Federer.

"In America we have so many wonderful stadiums, you figure it's normal they would be taking the lead with something like this," Federer said.