Since his debut in May, Thien needs to reach at least the third round this week to avoid falling out of the top 20, according to the ATP. The 20-year-old reached the second round in Paris last year.
The American won his first ATP title at Metz 2025 and reached the semifinals in Montreal this season.
On Monday, Van Assche defeated Mariano Navone 6-3, 7-5 for his first win in Paris.
Rolex Paris Masters 2026 first round results
Lorenzo Musetti d. Hubert Hurkach 7-6 (3), 7-6 (10)
Luca Van Aschen d. Mariano Navone 6-3, 7-5
Student Tien (15) d. Adolfo Daniel Vallejo (LL) 6-3, 5-7, 7-5
Novak Djokovic (10) vs Valentin Vacherot (Tuesday)
Rinki Hijikata (Q) d. Luciano Darderi 6-2, 6-2
Thomas Martin Etchver d. Andrei Rublev 6-4, 6-7 (4), 7-6 (3)
Cameron Norrie d. Mateo Arnaldi 6-4, 1-6, 6-3
Matia Bellucci (Q) d. Lorenzo Sonego (Q) 6-4, 6-3
Arthur Rinderknech d. Francisco Cherundolo (16) 3-6, 7-6 (2), 2-0 def.
Tommy Paul (12) vs Holger Rune (PR) (Tuesday)
Daniel Merida d. Sebastian Baez (LL) 6-4, 6-2
Quentin Hallis d. Arthur Ferry 7-6 (6), 7-5
Jan-Lennard Straff (SE) vs Arthur Gea (WC) (Tuesday)
Casper Rood d. Juan Manuel Cerundolo (Q) 6-3, 6-2
Rafael Jodar (9) d. Alejandro Tabillo 6-2, 6-3
Nuno Borges (LL) d. Moise Kouame (WC) 6-3, 6-1
Karen Khachanov d. Denis Shapovalov 6-3, 6-7 (6), 1-4.
Ignacio bus d. Zizu Bergs 6-2, 6-4
Thomas Machak (Q) d. Kyrian Jacquet (WC) 7-6 (7), 6-1
Alex Molkan (Q) d. Tituan Drogue (WC) 4-6, 6-4, 6-4
Benjamin Bonzi (Q) vs Taylor Fritz (11) (Tuesday)
Brandon Nakashima (14) vs Matteo Berettini (Tuesday)
Hugo Humbert d. Bottic van de Zandshulp 7-6 (5), 6-7 (9), 6-3
Alex Michelsen vs Jiri Lehetska (Tuesday)