Rafael Nadal could become the first player to begin the defense of his Wimbledon title under cover today 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 No. 1 Nadal, at least, can safely assume he will start on time thanks to Centre Court's sliding roof.

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

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

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.

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-time champion Roger Federer.

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

"But then again, Wimbledon is a different center 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."