The fifth seed won 6-3, 3-6, 6-2 over the fourth seed, who dominatedthe national tennis championships for several years before slipping downthe rankings earlier this year.
In the next match, Ho Chi Minh City's Thien, the national runner-up, will play the top seed, Pham Minh Tuan of Da Nang.
Tuan on December 26 beat Tran Hoang Anh Khoa of Military 6-3, 6-3.
Host athletes Quan and Khoa will play in later matches of the group.
In Group B, Le Quoc Khanh retired in the third set. He was 3-6, 7-5,3-1 down against his Hanoian rival Trinh Linh Giang before he quit.
In the other match, Military representative Tran Thanh Hoang came frombehind to win 5-7, 6-3, 6-2 against his teammate, Ho Huynh Dan Mach.
Khanh will face Mach on December 27, while Hoang meets Giang.
In the women's competition, the higher-ranking seeds earned easy victories in the second match of the group round.
Huynh Phuong Dai Trang of Ho Chi Minh City beat Le Thi Thanh Thuy ofKien Giang 6-0, 6-0, while Huynh Phi Khanh won 6-2, 6-3 against TayNinh's Tran Thi Thanh Thanh.
Meanwhile, Tran Thi Tam Hao defeated Dao Minh Trang 6-2, 6-2, while Nguyen Ai Ngoc Van trounced Si Boi Ngoc 6-4, 6-2.
The Masters, an end-of-year tournament, offers a total of 280 millionVND (13,300 USD) in prize money. The cream of the crop will be selectedfor the country's David Cup and the Fed Cup qualification rounds.
The finals are scheduled for December 31. The organisers plan to openthe event to regional competitors in the near future.-VNA