Andre Agassi told AFP on Tuesday that "incredibly gifted" teenage American Learner Tien has the raw ability and mental steel to become one of the top tennis players in the world.
The 19-year-old Tien reached the final of the China Open in Beijing this month and last week pushed former No 1 Daniil Medvedev to three sets at the Shanghai Masters before being beaten in the last 16.
Speaking in Hong Kong, retired great Agassi spoke glowingly about his young compatriot, who is widely regarded as one of the rising stars of men's tennis.
"He has learned the game phenomenally well. He is incredibly gifted, not just with his game, but with his mind," said the former world No 1 and eight-time Grand Slam champion.
"He has to overcome certain physical limitations versus other guys, as far as size and strength (goes), but he's still so young.
"When I really look at him from a tennis player execution standpoint, he's one of the few guys I say to myself, (I don't know) how would I help him get better – he does everything."
The 55-year-old Agassi, in Hong Kong for the Prudential NextGen Aces event, said he was particularly impressed by Tien's cool in the heat of battle.
Comparing him to Australia's world No 7 Alex De Minaur, who is seven years older than Tien, Agassi said: "My gosh, the thing I respect the most, it's like you look at him, you look at De Minaur, they're guys that just find a way to always show up, blood pressure never changes.
"They deliver the fact that they're going to make you beat them, and they have a level where they can raise their game and challenge the top 10 in the world."
He added of the Californian Tien, who only turned professional in 2023 and is currently ranked 38th: "Mentally he absolutely has what it takes."