Nine-time winner Rafael Nadal remains among the favourites for this year's French Open -- but Roger Federer is stronger than ever at 35, according to Spanish great Arantxa Sanchez-Vicario.

Sanchez-Vicario said Nadal's fitness would be the key at the season's second Grand Slam, starting next month, after he missed last year's French Open during an injury-hit 2016.

However, the 30-year-old Spaniard recovered to stun many observers by reaching January's Australian Open final against Federer, where he lost in five sets to his long-time rival.

"Well, definitely. I think that he is one of the favourites to win again at Roland Garros," Sanchez-Vicario, a four-time Grand Slam-winner, said at this week's Rendezvous Roland Garros event in New Delhi.

"Hope he is not injured. So that's the most important (thing) for him in order to compete well again," added the 45-year-old.

Nadal, currently ranked seventh, has lost three times to Federer this year after this month's Miami Open final and their clash in the last 16 at Indian Wells.

Sanchez-Vicario was full of praise for the evergreen Swiss, who won a record-extending 18th Grand Slam title at the Australian Open.

"He has become stronger. His backhand is powerful and he is winning many points there. And he has that mentality in the game, he is a champion," she said of Federer.

"It's great for sports, and for tennis, to see him doing so well again and winning Grand Slams at that age. He is a legend and a champion."

Sanchez-Vicario, who was just 17 when she beat German great Steffi Graf to win the 1989 French Open, added that she was open to the idea of joining the likes of Ivan Lendl, Boris Becker and Amelie Mauresmo as one of tennis's celebrity coaches.

"For me, you know, it (coaching) didn't happen because I have two kids, I am being a mom... and I am very happy doing that," said Sanchez-Vicario, who retired in 2002.

"Now that I am getting more involved in tennis, I leave the door open and could be one opportunity maybe in the future or as well as helping, consulting or trying to help some other players to be able to improve. So, I will see," she smiled.