'Federer incredible, but Serena the best ever' -- Mary Pierce

'Nadal almost unbeatable at the Roland-Garros' - Pierce (3:30)

Mary Pierce talks about her favorite players ahead of the French Open and about the return of Serena Williams and Maria Sharapova. (3:30)

Roger Federer recently tried to put to bed one of tennis' most wildly contested debates by suggesting that Serena Williams is in fact the greatest player of all time. While that may not end lay all talk to rest, two-time Grand Slam champion Mary Pierce concurs with Federer's view.

Pierce spoke to ESPN in India on a variety of topics, including Maria Sharapova's return from a doping ban, partnering up with a 16-year-old Sania Mirza, and the future of women's tennis.

"If you do look at the number of Grand Slam titles and who has more, it's Serena Williams," Pierce said. "It's really impressive what she's done, winning 23 Grand Slam titles and Roger has 20. So I would have to agree with Roger, even though he's incredible and what he does on court you can't even teach."

Sharapova is another player who is no stranger to picking up titles, but Pierce believes the Russian is still a little rusty following a 15-month ban which ended last year.

"You know Maria just doesn't seem the same," Pierce said of Sharapova, who beat 2017 French Open champion Ostapenko in Rome last week.

"In many ways on the tennis court when she's competing, there's just something missing and you can't really put a finger on what it is but that's what's making the difference and that's why she's not having the results that she used to.

"It's hard for her to come back from something like that and being out for so long. I can see it when I'm watching it, there's just a little bit less in every area of the game and that's what's making the difference."

From decades together of enduring rivalries etched in people's minds, women's tennis has now moved to an open field, but Pierce doesn't necessarily see it as a bad thing. Of course, she does feel for the average tennis fan who'd love an engaging match-up.

"Serena is dominating, but there aren't many rivalries. It seems like people miss that and people want to see a rivalry between two players, but right now women's tennis is what it is. It's pretty much an open field, so you get to a Grand Slam and you don't know who's going to win. So now's there's a lot of interest and excitement on what's going to happen, who's going to win this match and who's going to win this tournament. So it's different."

It also means that there are no clear favourites. Only last year, unseeded 20-year-old Latvian Ostapenko became the proud holder of the French Open title. Pierce is of course wise to steer clear of the trappings of picking a favourite among the women for this year's title.

"I don't think I can choose one name because it's quite a handful. Simona (Halep) is the No. 1 seed, clay is not Serena's best surface and she's not in top form yet, (Elina) Svitolina looks very good, Ostapenko is defending her title and (Karolina) Pliskova is playing well so we'll see."

In the men's contest though, she has a fairly easy task picking a winner.

"It's very hard to go against (Rafael) Nadal. He's amazing on clay, and has been looking great lately. When he gets to Paris something switches on in his mind and game and he's almost unbelievable."

Pierce, the most recent French player to win Roland Garros in 2000, also subscribes to the view of it being the hardest Grand Slam to win. It's a tournament she always loved to watch and dreamed of winning. When she finally did lift the trophy, it was 'all the emotions you can possibly imagine at one time', as she puts it.

"It's a really challenging Slam. The clay court slows the ball down and you need to hit a lot more shots to win a point. So you need to be patient and physically very fit and strong. You know strong legs, strong cardiovascular to move around, hit the balls back, stay in a long rally and last for a long time. Also you need to have that mental strength and emotional reserve to dig deep, match to match especially as it comes to an end."

Born in Montreal to a French mother, Pierce was one of the hardest hitters on the women's tour in her time and she has little doubt that she would fit in well even in today's scenario which revolves around the power game.

"I remember Monica Seles coming on the tour and she had two hands on both sides. She had incredible power and incredible aggression and I think that began to change things, revolutionize tennis and brought the swinging volley or what some call drive volley. With technology coming into rackets and strings improving, the balls are coming off much faster and with much more pace so definitely the game has changed in that aspect. Everything is much faster. Since I was one of the power hitters then, I think I could still do very well if I played in today's era."

Pierce is no first-time visitor to India. At the 2003 WTA India Open in Hyderabad, she ran into a 16-year-old Sania Mirza. They went on to pair up for doubles in the tournament and she knew back then that the prodigious teen would go a long way.

"I was asked if I would play doubles with a young local girl and I said sure why not. I practiced with her before the match and I thought 'wow this girl smacks the ball, she hits the ball so hard so flat, I haven't seen anyone hit that hard before'.

"So we played our first-round match. It was at night, it was a packed stadium and we won. It was crazy. I thought 'if this girl can keep the balls in the court (because a lot of the times they would fly out) she would be great one day'. She went on to be No. 1 in doubles and I'm not surprised."