In the Rios book Safin said Rios had the talent to win 10 majors. (Marcelo Rios: The Man We Barely Knew)
Thomas Johansson said Rios could make you feel like a beginner.
Bollettieri said Rios was his biggest disappointment. Rios was a great player but a bust in terms of winning majors. Just wasn't strong enough physically. The talent was there though.
"He looked like he was trying to imitate Sampras when I first played him." Agassi in Facing Federer