Rafael Nadal insists Novak Djokovic remains a major threat to his hopes of an 11th French Open crown.
Serbian Djokovic has struggled to find his best form during a difficult few months having undergone elbow surgery.
The former World No.1 is the 20th seed at Roland Garros, but he showed glimpses of his old self at last week's Italian Open before bowing out to Nadal in the semi-finals.
Nadal, who has lost just twice in 81 matches on the red clay of Paris, is once again the heavy favourite to triumph.
But he said: "Novak is one of the best players in the history of this sport. My answer is he's not coming back, he's always there.
"Sometimes when you come back from injuries it's difficult to play that well from the beginning, but he played a great event in Rome and is no doubt one of the candidates to win here."
Nadal, himself only recently back from a hip injury, went on to win in Rome and begins his French Open campaign against Ukraine's Alexandr Dolgopolov.
"I feel good," added the Spaniard, who is back at the top of the world rankings. "Of course, after a very tough beginning to the season, with two injuries in the same place, I came back and played well.
"I have played a lot of matches and had good success since I came back.
"But every tournament is different, and here we are to try our best, to have good days of practice before the tournament starts, and try to be very competitive for the beginning."
Alexander Zverev is another expected to challenge Nadal's dominance, the 21-year-old German taking him to three sets in the Rome final and winning in Munich and Madrid this year.
Zverev is seeded second and faces unfancied Lithuanian Ricardas Berankis in round one.
"It's obviously been a fantastic clay-court season for me," he said. "Winning so many matches in a row was great coming in here.
"Obviously there are a lot of other great players playing here; Rafa, Novak and everybody. They are all getting on top of their game.
"This is a long tournament with a lot of hard matches. I'm not trying to think that I'm going to play Rafa in the final. That's not how I'm thinking.
"I'm thinking about every single match.
"I'm thinking about how to beat Berankis in the first round. That's my thought process right now."
Meanwhile, former Wimbledon quarter-finalist Bernard Tomic of Australia - who is now ranked a lowly 208th in the world - reached the main draw after winning his final qualifier.
Only four months ago, Tomic was appearing as a contestant on the Australian version of I'm a Celebrity... Get Me Out of Here.
But the 25-year-old will be back on more familiar territory next week following a 7-6 (7-5) 7-5 win over Portugal's Goncalo Oliveira.
Britain's Cameron Norrie failed to make the final of the Lyon Open after a 6-1 7-6 (8-6) defeat by experienced Frenchman Gilles Simon.
Norrie (22) had claimed the biggest win of his career by beating World No.10 John Isner in the quarter-final stage, but the World No.102 was beaten in straight sets on the clay-court by 75th-ranked Simon.
Norrie struggled with his serve in the first, but took the second to a tie-break before blowing two set points.
Simon will face Serbia's Dusan Lajovic or top seed Dominic Thiem in the final.
British No.3 Norrie is expected to rise up the rankings after a positive week in France, reaching a ranking of 85th as he enters the French Open next week.
He had never played a professional match on red clay until he featured in the Davis Cup in February.
But after a first set in which he lost his serve three times, the left-hander fought his way back into the semi-final.
The second set played out without a single break of serve as Norrie matched home favourite Simon, before the match slipped away in the tie-break.
Elsewhere, James Ward is through to the semi-finals of the ATP Challenger Tour in Loughborough after Russia's Teymuraz Gabashvili retired in the second set with the Briton leading 7-5 5-4.