In 1974 Rosewall made the final of Wimbledon and the US Open but WAS NOT allowed a rest day between the semi-final and the final. Plain and simple - Rosewall was stuffed. I have no doubt Connors would have won both matches but there is no way he'd have won so easily if Rosewall had been given adequate rest.
I also have no doubt that, were Rosewall in his prime, he would have beaten Connors on one of those two occasions - most likely at the US Open - if not both times. Despite what someone here said, Rosewall didn't have a soft serve - there was a fair bit of weight on it and it was always very deep-, he just didn't have a lot of velocity. However, given that we all know Connors thrived on pace, Rosewall was the player most likely to give him fits. That he didn't was ONLY due to him being 40 (close enough) and not able to recover without a rest day.
That Connors played well on those occasions is sure : at the time he was very confident and hadn't suffered the bad losses he suffered in later years. Jimbo had almost nothing to lose whereas it was Ken's last occasion to win Wimby in the first final and a Slam tourney at Forest Hills.
Besides Connors had easier draws in both events than Kenny and as AndrewD recalled Ken had no rest day in both tournaments. However I think that a rested Rosewall would have lost something like 63 63 63 to Connors then in his prime.
- I don't know if the best Rosewall (1962-1963) would have beaten the best Connors (1974, 1976) but there are glimpses of information :
Apparently the best Rosewall in his 40's was the one who played at Sydney indoor in October 1977 : he beat Pfister who was a young hard hitting player, he overcame Gerulaitis who was then close to his very best (#4 or 5 in the world) 76 64 and for once he played well enough against Connors and lost 75 64 62 to Jimbo. If an almost 43 years old player was able to take 11 games from a very top player we can guess that at 28-29 years old, Kenny would have been able to won many more games.
In September 1972 Connors wasn't at his very best but was already a very good player circa the 15th place in the world (who earlier had beaten players like Smith, Graebner, Froehling all in 1971 and Hewitt, Orantes, in 1972, ...) : Rosewall, though past his prime, easily beat Connors 63 62.
Rosewall and Connors had some comparable strengths : strong backhand, return and speed.
I think that the last one was very important.
We can guess that in the mid-70's Rosewall's speed was inferior to that of his peak days and I suppose that it made a very big difference.
Let's suppose that Ken's speed in the mid-70s was only 5% slower than his peak speed : it is sure that the difference (loss of speed) made many more than only 5% more games lost.
And on clay I think that the best Rosewall would have much bothered Connors on Jimbo's often erratic and weak forehand approach.
So I can be wrong but I think that the best Rosewall (early 60's) would have been very close to the best Connors (mid-70's).
About Ken's serve, I think that both GPG and AndrewD are right.
Rosewall had a "soft" serve especially if we compare his with Hoad's, Gonzales's, Newcombe's, Smith's or Tanner's ...
However, Laver for instance when he faced Rosewall for the first time in January 1963, was amazed to note that Rosewall's serve was much heavier and quicker than he thought before as a simple spectator.
At his best Rosewall's serve had, as AndrewD wrote, a fair bit of weight and was often very deep and well placed. In particular Ken's second serve could be very sure and good. But it is right that both serves (especially the second one) lacked velocity.
Other point about his serve. Kenny being small, his serve was low and it often surprised and bothered the returner.
So when Rosewall was "hot", his serve wasn't open to attack and in that case Kenny wasn't far from victory
but we must recognize that when Rosewall was down, his serve could be attacked and then defeat wasn't very far.
Almost the same can be said about his forehand which was less good than his backhand.
Someone (I don't remember who) said that at his peak (during his best pre-open years) Ken's forehand pace and timing were perfect.
In his amateur days his forehand pace and timing weren't that good but Ken perfected them in the pro circuit. But when he became older (in the open era) and therefore less fit he began to lose occasionally the good pace and timing on his forehand which would explain some faults on that stroke.
So in conclusion his serve (and forehand) could be weak when Rosewall wasn't at his top and players like Sedgman, Trabert, Hoad, Gonzales, Laver, Newcombe, Connors could easily handle Muscles on those occasions but whenever the little Master was inspired
(Adelaide (Gonzales tour) 1957, Coubertin 1959 (European tour) Wembley 1960, Forest Hills 1963, Paris 1963, Paris 1964, Wembley 1964 (though he lost), US Pro 1965, Paris 1965, US Pro 1966 (he lost), Wembley 1967 (he lost), Roland Garros 1968, Forest Hills 1970, Dallas 1971, Dallas 1972, and other occasions I forget) everyone had to fear the Sydneysider player because both his serve and forehand could be good weapons (sometimes his forehand was as efficient as his backhand) added to his other great strengths, and then victory was very close.
