INDIAN WELLS – Rafael Nadal faces the toughest test yet in his injury return as the ATP Masters on the hard courts of Indian Wells brings the "big four” of men’s tennis together again.Not since last year’s Wimbledon have world No 1 Novak Djokovic, Roger Federer, Andy Murray and Nadal all been entered in the same tournament.Nadal, who won the Indian Wells title in 2007 and 2009, will be under the fiercest scrutiny as the fifth-seeded Spaniard continues his return from a seven-month injury absence that kept him out of last year’s US Open and prevented him from defending his Olympic gold.His February appearance at Vina del Mar, Chile, marked his first tournament since a shock second-round exit from Wimbledon, after which he was idled by his left knee injury.Nadal, whose 2012 French Open victory was his 11th Grand Slam title, reached the final at Vina del Mar and went on to claim clay court titles at Sao Paulo and Acapulco.But even he admits he doesn’t know how his knee will respond to the rigors of a hard court Masters event, where the field features 49 of the top 50 players in the world.The last time Nadal played a hard court event was at the 2012 Masters in Miami, where he pulled out of his scheduled semifinal against Murray with left knee trouble."All I can do is all the right things to be ready,” Nadal said after a light-hearted exhibition in New York on Monday. "I really hope that the knee will go well. I don’t know how well, but the knee will give me the chance to compete."I know the knee isn’t 100 percent better yet, but if it is now well to be competitive, to run with no limitations or limitations on my movements, that’s a lot.”adal, seeded fifth, could meet defending champion and second seed Federer in the quarterfinals.To get that far Federer could have to get through a re-match of last year’s final against American John Isner in the fourth round.  World No 1 Djokovic and No 3 Murray are on a semifinal collision course. Djokovic, riding a 13-match unbeaten streak in 2013, vanquished Murray in the Australian Open final in January and has since claimed the title in Dubai. The Serb is seeking a third Indian Wells crown after victories in 2008 and 2011.All 32 seeded players enjoy first-round byes, so Thursday’s opening day of action saw players jockeying for the chance to take them on.