On average ATGs play better than non-ATGs.
Djokovic and Nadal in percentage played more ATGs than Federer. That's why they had it tougher IMO.
On average, older ATGs lose to younger ATGs. Since 1980, 30 year old+ ATGs are 7-31 against younger ATGs (3 years older+) in slams QF/SF/F. If you count Hewitt and Murray, 11-33. If you count just Murray, 10-32.
When facing younger all-time greats (at least 3 years younger)
• Federer: 3W—8L
• Connors: 3W—11L
• Agassi: 0W—3L
• Lendl: 1W—6L
• McEnroe: 0W—3L
If you count Hewitt and Murray as ATGs, it’s 11 W, 33 L.
• Federer: 6W—9L
• Connors: 3W—11L
• Agassi: 1W—3L
• Lendl: 1W—6L
• McEnroe: 0W—3L
• Sampras: 0W—1L
—Connors beat Lendl, 1982 USO F
—Connors beat Lendl,1983 USO F
—Connors lost to McEnroe, RG 1984 SFs
—Connors beat Lendl, 1984 Wimby SFs
—Connors lost to McEnroe. 1984 Wimby Fs
—Connors lost to McEnroe, 1984 USO SFs
—Connors lost to Edberg, 1985 RG QFs
—Connors Lost to Lendl, 1985 RG SFs
—Connors Lost to Lendl, 1985 USO SFs
—Connors Lost to Becker, 1987 RG QFs
—Connors Lost to Lendl, 1987 USO SFs
—Connors Lost to Agassi, 1988 USO QFs
—McEnroe Lost to Edberg, 1989 Wimby QF
—Connors Lost to Agassi, 1989 USO QFs
—Lendl Lost to Edberg, 1990 Wimby SFs
—Lendl Lost to Sampras, 1990 USO QFs
—McEnroe Lost to Sampras, 1990 USO SFs
—Lendl Beat Edberg, 1991 AO SFs
—Lendl Lost to Becker, 1991 AO Fs
—Connors Lost to Courier, 1991 USO SFs
—Lendl Lost to Edberg, 1991 USO SFs
—Lendl Lost to Edberg, 1992 AO QFs
—McEnroe Lost to Agassi, 1992 Wimby SFs
—Lendl Lost to Edberg, 1992 USO QFs
—Sampras Lost to Hewitt, 2001 USO, Fs (if you count Hewitt)
—Agassi Beat Hewitt, 2002 USO SFs (if you count Hewitt)
—Agassi Lost to Federer, 2004 USO QFs,
—Agassi Lost to Federer, 2005 AO QFs,
—Agassi Lost to Federer, 2005 USO F
—Federer Lost to Djokovic, 2011 USO
—Federer Lost to Nadal, 2012 AO
—Federer Beat Djokovic, 2012 Wimby SFs
—Federer Beat Murray, 2012 Wimby Fs (if you count Murray)
—Federer Lost to Murray, 2013 AO, SFs
—Federer Beat Murray, 2014 AO QFs (if you count Murray)
—Federer Lost to Nadal, 2014 AO SFs
—Federer Lost to Djokovic, 2014 Wimby Fs
—Federer Beat Murray, 2015 Wimby SFs (if you count Murray)
—Federer Lost to Djokovic. 2015 Wimby Fs
—Federer Lost to Djokovic, 2015 USO Fs
—Federer Lost to Djokovic, 2016 AO SFs
—Federer Beat Nadal, 2017 AO Fs
—Federer Lost to Nadal, 2019 RG SFs
—Federer Beat Nadal, 2019 Wimbledon SFs