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Laver (centre) with the other greats.
Laver is not only considered by many as the greatest tennis player of all time due to his two Calendar Slams, but his doing so on the second turn at age 31, 7 years after he was banned from the Slams by turning pro.
During what was arguably his prime years, he was able to win 5 U.S. Pro Championships, and in 1967, a year before the Open Era began, Laver won all three of the unofficial Pro Slams, Wembley, U.S. and French, and also winning the Wimbledon Pro, which essentially gave him his other Calendar Slam.
It is entirely within reason to assume Laver, had he been able to participate in the "official" Slams, would have racked up close to 20, as noted by his Pro Championship wins between 63 and 67.
It would be hard for anyone to match or overtake Laver.
The one argument that could be used to diminish Laver's accomplishments is the one that can be used for any athlete in any sport, lesser pool of talent.
Although this may be true, it does not take away from Laver's dominance. Nadal's road to surpassing Laver would start with him winning the 2011 Australian Open giving him the first 4 consecutive Slam victories since Laver, albeit not in the same year.
Laver's all court dominance is dully noted and well respected, however he in fact did not win a single slam on hard surface, as all the slams were played on grass or clay.
Nadal has much work to do with Laver's 17 Slam Finals and his immense success in the pro years before the Open era.