I would not call them VERY strong. Perhaps a little long in the tooth.
Rosewall certainly did well in 1963 Stade Coubertin against Laver, who, in his own words, played the greatest tennis of his life in that match.
Dan, Most of Rosewall's opponents were actually very strong: Laver, Gonzalez, Sedgman, Hoad etc. I concede that players like MacKay and Ayala were not quite as strong but still strong. Even Mike Davies and Robert Haillet were fine players.