The first round of the French Open begins on Sunday May 28 from 11:00am.
Murray could face unsafe Juan Martin del Potro in the third round.
Victoria Azarenka, the 2012 and 2013 Australian Open champion, hasn't played since last year's Roland Garros after the birth of her son.
"They are so good, no, they're going to be fighting for the most important things and I am sure that they're going to be fighting for Roland Garros", said Nadal of Murray and Djokovic.
However, with defending champion Novak Djokovic and world number one Andy Murray both struggling for consistency, it appears to be a thankless task. "But the last two months, I just haven't played well", Murray said, speaking at an event in the French capital by his sponsor Under Armour.
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Just as Djokovic failed to maintain his level after completing a career grand slam at Roland Garros previous year, Murray has admitted to a drop in motivation since climbing to the top of the world rankings.
The 23-time Grand Slam victor learned she was pregnant just two days before starting a successful assault on January's Australian Open and won't return before next year.
Indeed, Djokovic has shown some vulnerability on clay too, recently losing to Alexander Zverev in the final of the Italian Open.
"Rafa, next to Roger, has been the best player so far this year in terms of results", he said. That's why I'm very excited for him being here, because it's a great opportunity for me to learn.
An elbow injury forced him to a take a short break and a bout of shingles didn't help but Murray's greater concern is the inability to reproduce his renowned agility around court. The two were spotted together on the tennis court and the first photos of the stars were published on social media.
In the women's draw, top seed Angelique Kerber starts her French Open campaign against Ekaterina Makarova of Russian Federation. "I was talking with Andre for a couple of weeks during Madrid and Rome on the phone, kind of preparing for the matches, analysing matches".
Muguruza has a tough opener against former champion Francesca Schiavone.
Title favourite Simona Halep, who is battling an ankle injury, was drawn in the same quarter as Ukrainian Elina Svitolina, who beat her in the final in Rome last weekend.
Garbiñe Muguruza is the reigning women's singles champion, although Serena Williams has made an appearance in the finals in three of the past four years.