Best Men's Player to have never won a Slam would be :
Marcelo Rios :
Reached Number 1
Lost his only Grand Slam final to Petr Korda ( who would test positive for a banned substance a couple of months later)
Was widely regarded the best tactician on tour in the late 1990's.
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One of the problems with selecting Rios as the best of those never having won a major is that his alleged skills as a tactician usually weren't enough to help him overcome Top 10 opponents in majors. He defeated only two of them, Enqvist, #9 at the 97 Australian and Bruguera, #8 at the 97 US. That's a very poor showing for a player who spent some time ranked #1. He did, however do better at Masters events, especially in 1998.