-- Roger Federer, who turns 33 in August, is the all-time leader with 17 Grand Slam singles titles. He's three ahead of the retired Pete Sampras, but there is a serious threat looming in the distance.
Rafael Nadal, nearly five years his junior, already has 13 majors under his belt. Eight have come at the French Open, which begins play Sunday. Here's how the experts see the race between two of the greatest players ever:
Steve Flink has covered tennis for 40 years and is a columnist for TennisChannel.com: Most of the best guys -- [Stefan] Edberg, [Andre] Agassi, [John] McEnroe -- weren't getting to double digits, so I didn't think Roger and Rafa could do it. Maybe somewhere between seven and nine, but what they've done is amazing. The 9-2 [head-to-head in Slams] accurately reflects the rivalry. Rafa knew what he wanted to do against Roger and, very early, got used to the idea of beating him. Roger's never solved the riddle. Had Roger managed to send him off on a few occasions, he'd be in a different spot. I don't think Rafa can catch him if Roger wins one more. He has to stay where he is. I think with Roger playing the way he's playing now, Wimbledon's the only one he can win, particularly if he avoids Nadal. But if Federer doesn't manage to win Wimbledon, Rafa can chip away at this. I can see him potentially tying -- but I can't see him breaking it. Maybe he wins four more.
Stanislas Wawrinka, ATP World Tour No. 3: Yeah, for sure [Nadal] can [catch him]. It's going to be tough because 17 is a lot of Slams, and maybe Roger will still win one more or even more than that. It's going to be interesting. It's good for tennis. It's interesting for the fans. It's going to be great to watch.
Mary Carillo, analyst for NBC and Tennis Channel: The big surprise with Rafa is the last couple of Wimbledons. I thought he got his grass-court game to a better place, but it has defeated him as he's aged. As he gets older, the French takes more and more out of him. It's an incredible drain, mentally and emotionally. There's more pressure on him in Paris than anywhere else. The last year few he's showed up exhausted at Wimbledon. OK, I've arrived at my conclusion: I think he has more in him than Roger.
Nick Bollettieri, soon-to-be Hall of Fame coach; worked with 10 No. 1-ranked players: The whole thing depends on if Rafa plays the way he does, giving 150 percent all of the time. This year he has to win at least one Grand Slam. If he does, I think he'll be around for another three, four years. If he stays healthy and begins to put the fear back into playing against him, he can win a lot more Grand Slams. They used to be afraid to play Nadal, but now they think they can do some damage.
Brad Gilbert, ESPN analyst: Will Rafa catch him? Your feeling may depend on the most recent result. If you were told he'd win the  Aussie, I might say yes. Four is a tough number. I'm sure if he perseveres, it will be close. One Slam is a good year, two unbelievable.
Justin Gimelstob, Tennis Channel analyst: I'd be surprised if he didn't (catch Roger). For all these players, my first hope is they stay healthy. I suspect Rafa, Novak [Djokovic] and Murray will cut into each other's stashes. Put [Juan Martin] del Potro into that grouping as well. As dominant as Nadal is on grass, he's viable in all of the Slams next four or five years. We all know the deal with the knees. I'd be hard-pressed to think he doesn't have four, five more in him -- gosh he might have four, five Frenches.
Paul Annacone, former Sampras and Federer coach, current coach of Sloane Stephens: I wouldn't bet against Roger adding to his number and obviously Rafa's significantly younger. If he stays healthy, he's got a number left. It's a cliché, but it's interesting to see how things ebb and flow. Does Roger finish with 17 or win one more or even three more? He's definitely always a favorite for Wimbledon. But at his age, sustaining it for two weeks is the challenge. Rafa's always the favorite, healthy or not, to win the French. I'm most curious how guys like Djokovic and [Andy] Murray evolve. Look at [Stanislas] Wawrinka; his results leave other players starting to believe. This time next year, it will be interesting to see where the two stand.
Peter Bodo, Tennis.com columnist: I'll go with the frontrunner, Federer. The older you get, the harder it is. I thought if Rafa had won the Australian, OK, that's doable. Now I'm not so sure. I'll give Rafa two French Opens right off the bat. After that? I'm not sure about the hard courts. Or Wimbledon, where he's been derailed a few times. Plus, I think Roger might get one more. All it takes is some luck. Djokovic gets hurt, Murray's not in form. He could get to 18, but he's going to need some help.
Pam Shriver, ESPN analyst: A few years ago when Roger was resting on 16, I said he'd get two more. He might be my pick for this year's Wimbledon. I think Rafa's going to continue the pattern where it's a knee injury or a sensational run. I think Rafa has figured out what Roger has known all along: Proper pacing is the key. How much to practice, how much hard-court play, etc. He's learned a lot the last two years. Personally, I think they're going to finish with the same number. It would be poetic and kind of cool for both of them to end on 18.