An undefeated WTF means beating 5 Top-10 players, which is hardly ever done in the majors. In fact- Borg, McEnroe, Connors, & Lendl preferred playing in the WTF rather than the Australian Open most of the times. Therefore the numbers look like this- Federer (22) with 5/6 undefeated WTFs; Sampras (14) with 0/5 undefeated WTFs; Nadal (13); Lendl (13) with 5/5 undefeated WTFs; Borg (12), etc. Of note, Djokovic moves up to (8) with 2/3 undefeated WTFs, joining Agassi & Connors & Wilander. Federer's number looks quite safe for now.