This is the highest position a Vietnamese player has ever ranked.
Previously, Nam's highest ranking was 385th, recorded in November 2018.
The result is attributed to the fact that the player triumphed at ITF M15 in the southwestern province of Tay Ninh, earning 45 points.
With the latest ranking, he remains the number-one tennis player in Southeast Asia and is far ahead of rivals in the region. Kasidit Samrej, a 21-year-old player of Thailand, is the second at 662nd.
Nam has made an impressive comeback after winning three titles at tournaments in Egypt and Mexico. Previously, Nam finished the M15 Chiangrai tournament series held in Thailand with a runner-up position.
He also successfully defended the gold medal in the prestigious men's singles event at the 31st Southeast Asian Games.
The promotion on the ATP rankings is expected to help him have more opportunities to participate in many international tournaments.