The four greatest at their peak for a hypothetical semi-final in four possible grounds :
GRASS
Federer
Sampras
McEnroe
Borg
...final Federer vs Sampras
HARDCOURT OPEN
Federer
Djokovic
Connors
Sampras
...final Federer vs Djokovic
INDOOR (hardcourt)
McEnroe
Connors
Lendl
Federer
...final McEnroe vs Federer
CLAY
Nadal
Borg
Kuerten
Federer
...final Nadal vs Borg
GRASS
Federer
Sampras
McEnroe
Borg
...final Federer vs Sampras
HARDCOURT OPEN
Federer
Djokovic
Connors
Sampras
...final Federer vs Djokovic
INDOOR (hardcourt)
McEnroe
Connors
Lendl
Federer
...final McEnroe vs Federer
CLAY
Nadal
Borg
Kuerten
Federer
...final Nadal vs Borg