Apart from the Australians in their Golden Era of the 50s and 60s, I don't think any players from the same nation have been fully supportive of their fellow countrymen.
Tennis is an individual sport. Rivalries between tennis players from the same nation are seemingly the strongest ones of all. On the surface, they all look chummy. But in reality, I think there is a lot of niggle between players from the same country when they are playing on the Tour. (In the modern game I do believe there might be some exceptions .. eg. Shapo and FAA .. but I think that happens when the players basically grow up together.)
It might be different come the Team Events where they play for each other. But on the Pro. Tour, I seriously think it is every man for themselves.
Connors and McEnroe simply took it to the next level because of their personalities.