Well Hoodjem Rios did get to #1 in the world and won five Masters Series. He just couldn't win a major, had trouble with the physicality of having to play seven 3 out of 5. I don't think he was physically and hence mentally strong enough to do it. One week tourneys were more comfortable for him to excel in.
In the book many players and tennis people call him a big underachiever, but then I read Vince Spadea say that he felt Rios was actually an overachiever. So, everyone has their opinions. Bottom line is Rios was a great player to watch.
"In the beginning my coach was angry at me because I was too nice on the court." --Gustavo Kuerten