According to this article in Forbes.com (click for entire article "US Open Win Would Boost Nadal Endorsement Career " Rafa winning the USOpen could land him many endorsement deals 
 Excerpt:

Perhaps more importantly, winning the event would catapult the Spaniard’s profile among corporate sponsors. He currently has deals with Nike, Kia Motors, Babolat and several Spanish firms. He makes around $15 million annually off the court from sponsors and appearance fees with Nike his biggest deal by far . This is just a fraction of the $35 million that his rival Federer makes from sponsors and exhibitions (Federer’s 10-year Nike deal is the biggest in the sport worth more than $10 million annually). Nadal ranked third overall in our recent look at the world’s highest-paid tennis players.

The top tennis players are an attractive bunch to pitch products for companies. Tennis fans are a wealthy demographic with high disposable incomes to spend money on apparel, equipment, cars and watches. Tennis is also one of the most global sports with tournaments spanning the globe.

The difference between Federer and Nadal is that Federer has moved beyond industry deals pertaining to rackets, shoes and apparel to sign global deals with Gillette, Credit Suisse, Mercedes and Rolex. Nadal has been limited to tennis deals as well as regional ones in Spain. Part of the issue has been Nadal’s limited English, but that is no longer the case as Nadal has looked very comfortable and smooth speaking to the press in his post-match commentary at this year’s Open. 

Also in Current sponsored news:
Richard Mille welcomes Rafael Nadal’s victory at US Open

On Monday 13 September 2010, Rafael Nadal won his ninth Grand Slam title, beating Novak Djokovic on court at Flushing Meadows in the final of the US Open, the only title which still eluded him. At just 24 years old, the Majorcan's record-breaking run continued as he became the youngest player of the Open Era to win all four Grand Slam tournaments. For the third time in a row, he held the winner's trophy aloft with his Richard Mille on his wrist. An ultimte watch, an extravagance, entirely in keeping with the great champion who wears it.