Rosewall won a pro grand slam (1963) and was clearly the best in the game from 1961 to 1963 and then was arguably the best in 1964/65.
And then numerous years as the game's second/third best.
He is up there with the best. Agassi isn't. One could argue that he was never the best in tennis in any year (though you could give him either 1995 or 1999).