Take a look here:
I don't know how tennis used to work those days, but they played some huge clay tourneys twice that year (or this is a bug on ATP's website?)
Look at the players Nastase won against in all those finals (Panatta, Borg, Pilic, Orantes, Emerson etc.).
I think it's the best season ever of a player on clay.
He played 12 clay tourneys and he won 9.
(1 final, 1 sf and 1 qf).
He won RG, Monte Carlo, Madrid, Barcelona (twice !), Rome, Cincinnati, Florence, Gstaad.
Also, the overall season is outstanding !
it is certainly one of the best. Nastase also won all '73 RG matches in straight sets.
i have not ever seen Nastase play on clay. i have seen him on other surfaces. he was very talented. seems like he should have won more bug events.
Compare with Vilas during 1977 and Nadal 2005
i agree with fsoica that Vilas faced a bit of a weak field. he was obviously a great player, but, imo, he had little chance to beat Borg if Borg had played RG. (similar to Federer vs Nadal at RG.)
at the time, there were competing tours. Borg played World Team Tennis, so he could not play in RG.
on clay in '77
Nice: Borg d Vilas 6-4, 1-6, 6-2, 6-0
Monte Carlo: Borg d Vilas 6-2, 6-3
RG '75: Borg d Vilas 6-2, 6-3, 6-4
RG '78: Borg d Vilas 6-1, 6-1, 6-3
Borg was 17-5 vs Vilas, most of them played on clay.