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 semi-final 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. That happened last week in Acapulco for the first week and that's going to give me a lot of positive feelings and confidence in myself."
Juan Martin del Potro, who played in the exhbition, was full of enthusiasm for his rival's return.
"We are so blessed to see him playing again," said del Potro, who predicted it wouldn't be long before Nadal was again challenging for number one in the world.
Nadal, seeded fifth, could meet defending champion and second seed Federer in the quarter-finals.
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 semi-final 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.
Murray hasn't played since falling to Djokovic in a grinding four-set final at Melbourne.
However, the Scot could overtake Federer at No. 2 in the world rankings if he reaches the final.
Fourth seed David Ferrer, who has edged ahead of fellow Spaniard Nadal in the world rankings, lies in the bottom half of the draw with Federer and Nadal, in the same quarter with sixth-seeded Czech Tomas Berdych.
Djokovic is projected to face eighth-seeded Frenchman Jo-Wilfried Tsonga in the quarter-finals, but Tsonga has a tough path to that point with American Mardy Fish, 12th-seeded Marin Cilic and big-serving Canadian Milos Raonic in his section.
Murray's projected quarter-final foe is seventh-seeded del Potro, the 2009 US Open champion who is seeking a first Masters crown.
All 32 seeded players enjoy first-round byes, so Thursday's opening day of action will see players jockeying for the chance to take them on.
Djokovic will open against either Fabio Fognini of Italy or Slovenia's Aljaz Bedene.
Federer will launch his title defense against either Denis Istomin or a qualifier while Nadal will play either American Ryan Harrison or Japan's Go Soeda.
Murray, who has crashed out in his Indian Wells opener the past two years, will face either either Evgeny Donskoy of Russia or Japan's Tatsuma Ito in the second round.