It's interesting to look at the Penultimate leaders.
Connors, Lendl, Sampras at 984, 908, 869, are all higher than Federer's 850, but then their last major came about 60, 90, 110 matches later respectively. Federer got his last major win close to 200 matches later, mostly due to Nadal and Djokovic dominating RG 2010- RG 2012.
I believe that one of Federer's greatest accomplishments was winning 2012 Wimbledon in match #1045, defeating Murray and Djokovic, and regaining the World #1 ranking when his strong next generation competitors, Djokovic and Nadal were in their prime peak years.
Much like Connors, he has demonstrated his strength over multiple generations:
1. He and his generation rose at the end of the Sampras/Agassi generation at age 19-21, helping to push them out.
2. He dominated at #1 over his own generation from Feb 2004 - Aug 2008, age 22-27.
3. He solidly competed with the Nadal/Djokovic generation from then on, never worse then #3, and even topped them at age 31, reaching #1, and was still #2-#3 in 2014-2015, at age 33-34.
4. The following generation, Nishikori/Raonic leading the way, has posed few problems until this injury ridden year when he lost to Raonic in the SF at Wimbledon.
5. The #NEXT generation, has shown some promise here and there, but is still unproven.
Federer looks like he has won his last major, but I won't rule him out till he retires.
Nadal looks like he has won his last or penultimate major, but ditto about ruling him out.
He has now played 976 matches. His last was at 834 matches, a 142 match difference.
Djokovic has played 894 matches. Ironically, his recent domination has added a lot of mileage. He certainly is within the range where players win their penultimate major, but his advantage is that there are no strong rising youngsters that have even won a major. However, the main challengers to him come from his generation, Murray, Del Potro, Cilic, and Wawrinka all with from 100-400 fewer matches in their bag. I see 2 maybe 3 majors more for Novak based on the historical match wins.
But who knows what will happen? Things can change fast in tennis.
Respectfully,
masterclass