Dear Joaquim Alves Gaspar,
from F.W. Krücken, "Ad Maiorem Gerardi Mercatoris Gloriam", Vol. 1, Münster 2009, 163, I take that the quote about the difficulty of representing the surface of the globe on a plane surface is in the "vita" of Mercator by Walter Ghim. The quote is as follows:
"sic quadraturae circuli respondet, ut nihil deesse videatur, praeterquam quod demonstratione careat".
I don't have access to the "Vita" but you might try to look it up. You should have a look at Krücken's work, by the way, which is collected in three (rather unexpensive) volumes published by Verlagshaus Monsenstein and Vannerdat in Münster (Germany).