Career winning percentage is a limited metric for evaluating players, because it tends to favor those - like Borg - who had relatively short careers. Borg quit the game just after he had been dethroned by McEnroe as world No. 1. Had he remained his career winning percentage would inevitably have declined, as it does for every older player. You can't have it both ways - giving Borg credit for what he "might have done" if he had remained in the game while at the same time touting his actual career winning percentage over an abbreviated career as evidence of his unique greatness.
For players with long careers the overall winning percentage is indicative but not definitive of greatness. Chris Evert is an acknowledged all time great who, among other achievements, has the highest winning percentage in the Open era. Despite this, the majority of tennis analysts would rank both Martina Navratilova and Steffi Graf higher, and some are also beginning to place Serena Williams ahead of her. The key issue is that players are judged primarily on the basis of titles won and high rankings attained. A high winning percentage indicates consistency, but it is valued primarily in terms of how this translates into these other accomplishments. Evert was no slouch in these matters, as 156 titles and 18 majors attest, but there are non-trivial reasons why most analysts would rank one or two Open era women players even higher.
Winning percentage over a player's peak years is a better criterion, since it indicates dominance. I'm not sure that Borg has much of an advantage over Federer here, however, since Federer's won-lost records for 2004 through 2006 were 74-6, 81-4 and 92-5 respectively. If you look at other indicators of dominance the case for Federer is even stronger. Between 2004 and 2007 he won 11 of the 16 majors played and the WTF three times. Of the twenty most important tournaments played over the period Federer won 14, was a finalist in three and a semi-finalist in two others. No other player, male or female, Open or amateur eras, has won 11 majors over 4 years. Federer also reached the finals of ten straight majors (and, eventually, 18 of 19 straight), was a semi-finalist in 23 straight and held the No. 1 ranking for a record consecutive 237 weeks. Peak Borg (1978-1980) was certainly dominant, but not to anything like this extent.
So we are left with claims that Federer's French Open win was a fluke (although he reached the finals on four other occasions) or that he faced "weak competition," which is always the last refuge of fans whose favorites' statistics do not stand up to comparison. I would take this last argument seriously if its advocates were to provide a set of testable criteria for evaluating competition across generations, but they never do so. What's left of their argument is pure assertion, which does not work very well.
