One of the arguments used by Roger Doubters, is that Fed has a losing record to a contemporary--Rafael Nadal. Head to Head, Nadal leads Federer 13 victories to 7. That number is somewhat inflated when we look at how those victories have come. Clay has always been a surface for specialists--meaning most players that are good on clay struggle on any other surface. There have been a few exceptions--Borg, Nadal--but for the most part, clay courters are 1 surface stars. On clay, Rafa leads Roger 9 to 2 in victories; they are tied at 2-2 on hard court, and Fed leads 2-1 on grass.
Many people believe that Rafa is the best clay court player in tennis history. There are plenty of goods reasons for that belief--he holds the record for most wins in a row on clay (81, ended by Fed), he won the first 4 French Opens he played in, and his clay court record since 2005 is 150-5, and at the French Open it is 31-1--and he's only 22. Roger has had to play the majority of his matches against Rafa on clay; if they had played 11 matches on grass, the record would certainly tilt Roger's way. By making it to the semi-finals at the French this year (which he went on to win), Federer tied a Roland Garros record with 5 consecutive semi's. So even though he has only won the title once out of those 5 tries, he's proven to be remarkably good and consistent on clay. A number of respected tennis greats believe Federer is one of the top 5 clay court players in tennis history, he just isn't good enough to consistently beat Nadal on the surface.
Federer's best surface is obviously grass--he's won 68 of his last 69 matches--so Nadal's Wimbledon victory is quite impressive. It seems like it's only proper that because Fed ended Nadal's record winning streak on clay, that Nadal returned the favor ended Fed's record streak of 65 straight wins on grass. Speaking of winning streaks by surface, Fed shattered the hard court record, by winning 56 straight matches (which wasn't ended by Nadal). The point of this, is that Nadal's head to head record doesn't do enough to Fed's legacy to legitimately dent his claim as the GOAT. Nadal is a great player--6 slams by the age of 22--and the fact that he has beaten Fed 13 times is remarkable. However, sports is very much about matchups and the Fed/Nadal matchup is nearly even, minus clay & grass, leaving hard court as the most neutral surface.
Some analysts have pointed out that although Fed has won 15 majors, guys like Laver would have won more titles without forced sabbaticals because of rules about professionals. It's true that Laver would have more than 11 titles if he had been able to play a full career; it's also true that when Laver played, 3 of the 4 slams were played on grass. If 3 of the 4 slams were played on grass today, Federer & Sampras would both have over 20 majors. Playing on grass so often would be a tremendous advantage for Roger, but that isn't the case today so other stats are needed to tell the tale.
Fed has now reached 21 consecutive semi-finals at the Slams, a monstrous record that more than doubles Ivan Lendll's second place record of 10 straight. 21 straight semi's is the most impressive record of Federers, it is the stat that towers above all others. For over 5 years, he has never missed the Final Four, he has never let minor injuries or sickness keep him down, he's never lost his focus, he's never let tight matches get to him, and he's never let an opponents best beat him. He made 10 straight Slam finals--the old record was 4--then he lost in the semi's (with mono), and now he's made 6 straight finals for the 2nd longest streak. Between the semi's streak, and the Finals streak, Fed has out distanced anyone, past or present, by a mile. In the last 6 years, Roger is 115-0 in Slams vs. anyone ranked outside the top 5. Fed has reached 15 major titles in a little over 6 years--it took Sampras 12 years to win 14. Fed had a 4 year stretch where he won 11 of the 16 Slams, something that no man or woman has ever done it tennis history. These are only some of the amazing stats that, along with 15 slams and the career slam, make Fed the greatest tennis player ever.
Comparing current players with past players is always subjective and difficult. I believe that today's athletes in every sport, are better individually. That individual improvement can be detrimental in a sport like basketball, but it makes sports like golf and tennis tougher. If the top 20 tennis players were put in a time machine and placed in a tournament--in their prime's--that would have everyone play everyone on every surface, I have little doubt that Fed would end up with the best record. Many tennis pros have said they believe he has the best all-around game they have ever seen, and I would agree. He is the most versatile player, with the greatest amount of skill to choose from when playing an opponent. No offense to Laver, but he would get worked in a tournament with guys like Fed and Sampras (Laver was 5-7, 130 pounds). A realistic analysis of the stats and records shows, conclusively, that Fed has the most impressive career resume, that he is the greatest of all-time, and that he will continue to pad his stats for at least a few more years.