I agree with Gizo that Lendl is very underrated. He's not Federer, but he should be ranked #1 in the 2nd tier great. Connors, Agassi, Mac doesn't doesn't hold tennis records like Lendl, who also was more consistent...multiple years with a 90+ winning percentage.
Tier1(goat): Federer; Tier 1.5: Laver, Sampras, Nadal, Borg;
Tier 2: Lendl, Connors, Rosewall; Tier 2.5: McEnroe, Agassi