That the old pro tour had lower percentages than other environments is something that's been long known
I think PC1 was one of the first to note it on this board, and I talked about it extensively in my more recent thread:
https://tt.tennis-warehouse.com/index.php?threads/career-percentages-of-17-alltime-greats.562591/
Emmo's win/loss percentage took quite a dive in '68 but again I think that was at least partly due to a change in formats, though the reverse of what we usually mean. Usually we say, correctly, that Laver's percentages went up in '68 because he was now participating in tournaments with large draws, in which he could pick up several easy wins over lower ranked opponents, in contrast to the old pro tour where it was common to play 2-round or 3-round tournaments with no easy wins.
Emmo had large draws throughout the 60s but in '68 he actually started playing smaller draws that were not that different from what he would have seen on the old pro tour. He was an NTL player and NTL often put on 4-man or 6-man tourneys. In those types of tournaments, again, you get at most 1 easy win, before you come up against the top players in the world.
I don't have Emmo's '68 record in front of me so I can't say at all how much this change in format was a factor in his low percentages. But I have to believe that it played some factor, because I'm sure he played at least a good number of small-draw events that he would not have seen on the amateur circuit.
So it's ironic, Emerson did, in a way, get a taste of the old pro tour format, but only when the sport went Open. That may sound strange but others including
@urban have pointed out that the transition to the OE was not sudden but somewhat gradual; the first years of the OE had many events that would not have been foreign to the old pros.
Emmo's amateur percentages were impressive, even higher than Laver's and Rosewall's. But we're looking at Emerson's absolute peak years (the mid-60s), whereas Rosewall and Laver had their best percentages as amateurs, before their peak years; and as OE pros, when they were in decline; so you can't do a true comparison.
One thing that's clear is that Emerson did not have great longevity, though that is not exactly news.