What do you mean by "superstar?" If you mean "great player," which is what the term means in other sports, then Lendl most definitely was a superstar. If you mean "mega-celebrity" or some such, then maybe not.
Anyhow, I think we've got three general areas to consider, as far as all-time greatness. 1) Career resume - just the stats, m'am. 2) Peak greatness - this is a mixture of stats, but also perceptions about how good a player was at his best. 3) Celebrity, mystique, and basically "sex appeal."
If we rate all 1-10, we might come up with:
Borg: 8/9.5/10 = 27.5
Lendl: 9/9/7 = 25
Borg comes out ahead. But if we take out the last part, which is wholy subjective, it is Borg 17.5 to Lendl 18 (and no, I didn't plan that).
(I'm just using these numbers to illustrate a point)
oh boy can you just put away these ridiculous feel-good about yourself ranking and stats? i know you do these things A LOT to seek some kind of credibility but honestly, it's getting meaningless.....
and you had Pioline as 'underachiever' from hear-say? what the hell? i know you really don't know about tennis but try and spend some time 'watching' it before you spill out something insane....please.....Pioline was an over-achiever as he simply didn't have it, at best he was a James Blake, athletic and generally good player but without James's firepower.
anyway you have put Lendl above Borg, then you retract, then you go back to say its debatable.....and you put in 'mystique' to try and argue a 'no' case, just cut it and stop with the mystique bull crap. Borg was top echelon because he won many RG/Wimbledon in the same year which was something considered as incredibly difficult (and it is indeed), as nobody has done that. you kept trying to justify yourself by saying he quit early, well just think about the context. In comparison to Lendl (who i thought was under-appreciated by idiot Americans), he played a shorter period yet still won more events that mattered.