Rafael Nadal could become the first player to begin the defence of his Wimbledon title under cover on Monday as rain threatens to dampen the start of the grasscourt Grand Slam.

If the forecasts prove accurate the opening day could be a busy one for the ground staff at the All England Club but world number one Nadal, at least, can safely assume he will start on time thanks to Centre Court's sliding roof.

- Nadal can beat Federer's Grand Slam mark

The Spaniard opens proceedings against American Michael Russell and is followed by Italian Francesca Schiavone against Jelena Dokic with Britain's Andy Murray's first round against Daniel Gimeno-Traver last on Centre Court.

Since the translucent roof, costing an estimated 80 million pounds ($130,000,000), was completed in time for the 2009 championships the weather Gods have been kind to Wimbledon organisers.

Glorious warm and sunny weather presided throughout the fortnight last year when the roof's only brief task was to allow the completion of a late Novak Djokovic match with the help of the structure's lighting system.

In 2009 the match between Amelie Mauresmo and Dinara Safina went down in Wimbledon history as the first to be played partly under cover after drizzle interrupted play.

Murray's epic against Stanislas Wawrinka on the same day was the first match to start and finish under the roof which weighs 1,000 tonnes and comes with its own air conditioning system.

Mainly, however, it has been redundant and for the vast majority of players involved in this year's draws the court conditions under the roof will be an unknown quantity, including six-times champion Roger Federer.

"In Halle I got the opportunity to play in some of the grass court matches under the roof," Federer, who begins his tournament on Tuesday, told reporters as heavy rain showers sent players scurrying from the practice courts at the weekend.

"But then again, Wimbledon is a different centre court, so definitely will take some getting used to in the beginning.

"It will be interesting to see. I honestly thought it was going to be a bit of a rainy Wimbledon this year. The spring was just too nice all over Europe it seems."

The few occasions that the roof has been used have brought mixed reviews, with some players claiming the humidity was stifling while others saying the grass became slippery.

Wawrinka became so hot against Murray two years ago that he came off looking like he had fallen asleep in a sauna.

"It's more humid. It slows the conditions down and the balls become heavier," Murray said.

Djokovic agreed that it did alter the playing conditions.

"I think when the roof closes it's a bit slower and a bit more slippery," the second seed told reporters. "But at least I know approximately what it feels like to be under the roof."

While the roof, which takes 10 minutes to close and enables play to re-start within 30 minutes, could finally start to earn its keep this year, organisers are quick to stress Wimbledon remains an outdoor, daytime event.

However, should it be raining 45 minutes before play is scheduled to start on Centre Court on Monday, the tournament referee is likely to hit the button and shut out the elements so the 125th championships can start on time.

($1 = 0.615 British Pounds)