Let me guess, you read the headline and instantly thought "hang on, Roger Federer has won 18 grand slams, whereas Rafael Nadal has won only 15"?
But does that necessarily end the argument? Is the total number of grand slams won the only way to judge which of these two legends of the sport is the greatest?
Let's begin by looking at the slams won by each player.
When assessing their major wins, Federer clearly benefited from having a few years before the advent of the "Big Four" as we know it.
Between 2003 and 2007, Federer won eight grand slams without beating any of Nadal, Novak Djokovic or Andy Murray along the way, and added two more in 2009 without playing any of those three.
A quick glance at Federer's final opponents in those majors shows the likes of Mark Philippoussis, Fernando Gonzalez and Marcos Bagdahtis, who are not exactly tennis heavyweights.
Nadal by contrast has had to beat one of the "Big Four" in all but two of his 15 grand slam wins, and in one of the ones when he didn't his final opponent was Stan Wawrinka, a three-time slam champion.
Secondly, Nadal is streets ahead of Federer in the head-to-head between the two players. He leads the overall record 23-14, and has won nine of the 12 slam meetings between the two.
Can Federer really be called the greatest ever when he has been consistently beaten by the man said to be the second best?
Finally, one of the arguments I often hear used to denigrate Nadal is that so many of his grand slam titles have come at the French Open, as if this points to him being some sort of one-trick pony.
But, let me put that another way. Isn't it remarkable that Nadal is even close to Federer's total given that only one of the four majors is played on a surface that is more suited to the Spaniard?
It remains subjective of course, but perhaps the argument is not as clear-cut as it seems.
Will the knees hold?
As tennis's dominant colour switches from red to green, it is worth asking which Nadal we might find ourselves watching over the forthcoming Wimbledon tournament which gets under way on July 3, asks the London Telegraph's tennis correspondent Simon Briggs.
The man for all seasons, who played in five Wimbledon finals? Or the grass-allergy victim who has not made it past the fourth round in SW19 since 2011?
The answer is hard to predict.
After thrashing Wawrinka last week Sunday to claim his 10th French Open title, Nadal was asked to rate his grass-court prospects.
In reply, he explained that he cannot strike his ground strokes properly without a stable base. And where the powdery clay cushions the impact on his cranky knees, grass wears him out by making him stoop for low balls.
"There has been a while since I don't play very good Wimbledon," said Nadal. "It's true that after 2012 what happened with my knees, [it has] been tougher and tougher to compete on grass for me.
"But if I have pain on the knees, then I know from experience that it's almost impossible. Because I need to feel strong, low, and powerful legs to play well in Wimbledon.
"If I don't feel that, then probably my chances are not there, no?
"We'll see how my knee behaves. Playing on grass is very special. You need to play at a lower level. The body posture is down. You have less stability on grass."
There is one reason for Nadalistas to feel optimistic. Just like his old rival Federer, Nadal has returned this season with a souped-up backhand.
Admittedly, he is not bashing as many clean winners off that wing as Federer. But he is taking the ball early and hitting fewer run-around forehands - two adjustments which reduce his step count and overall workload.
Nadal is meant to be playing a grass-court warm-up event in a week's time, the Aegon Championships at Queen's Club. This used to be a fairly regular stop on his itinerary. But he has appeared just once since 2011, because of the unfavourable British tax regime. He has to win the event just to break even, and in 2015 he lost to Alexandr Dolgopolov in the opening round.
Still, there is also the encouraging precedent of 2008 to point to.
That was Nadal's best French Open until this year (he dropped 41 games that season, as opposed to 35 over the past fortnight). And in that memorable summer, the switch to grass could not have gone more swimmingly. Nadal won Queen's before outlasting Federer in a Wimbledon final that is generally considered to be the greatest tennis match ever played.
This last comment might not seem like the boldest statement of intent. But then Nadal is never one to talk himself up. Where Federer's instinct is to expect the best, he fears the worst. "Doubts are good," Nadal said last Sunday, "because doubts give you the possibility to work with more intensity."
On Sunday night Nadal was asked if the top spot on the rankings ladder was a significant motivation. He shrugged. "[When you are] winning these kind of titles, then you have chances to become any number on the ranking. I am playing well. If I am able to keep playing well, why not?"
- The Daily Telegraph, London..