Monfils saved three of five break points and finished with a 21-13 edge in winners, including a 6-3 edge in aces. But Sinner was in control on both his first serve (winning 37 of 49 points, 75.5 per cent) and his second serve (17 of 23, 73.9 per cent).
Alexander Shevchenko of Kazakhstan ousted third seed Holger Rune of Denmark 6-4, 1-6, 6-3, and Tallon Griekspoor of the Netherlands did the same to fourth seed Hubert Hurkacz of Poland 6-7, 7-6, 7-6 in a match that took two hours and 34 minutes.
In other Round of 16 action, sixth-seeded Grigor Dimitrov of Bulgaria beat Hungary's Marton Fucsovics 6-3, 7-5, and Finland's Emil Ruusuvuori beat German Jan-Lennard Struff 6-3, 6-3.
IEB+ Argentina Open
Top-seeded Carlos Alcaraz of Spain fought through the first match of his title defence in Buenos Aires, defeating Argentinian qualifier Camilo Ugo Carabelli 6-2, 7-5.
Alcaraz received a first-round bye before taking the court Thursday to face Carabelli, who took a 2-0 lead in the second set. Alcaraz won the next five games, but Carabelli saved two match points en route to tying the match at 5 before the Spanish star pulled away.
Alcaraz's quarter-final opponent will be Italian qualifier Andrea Vavassori, who upset seventh seed Laslo Djere of Serbia 6-4, 7-5. Vavassori went 4-for-4 on break-point opportunities.
The second seed also fell in three sets, as Argentina's Federico Coria beat favoured Brit Cameron Norrie 6-2, 4-6, 6-3. Coria had five aces and saved seven of nine break points. In the final match of the day, Argentinian fifth seed Sebastian Baez topped Italian Luciano Darderi 6-4, 7-5.
Delray Beach Open
Third seed Tommy Paul rallied past fellow American Alex Michelsen 5-7, 6-4, 7-6 in a three-hour match in Delray Beach, Fla.
Paul smashed 15 aces and saved nine of 11 break points, while Michelsen saved six of eight. Paul's quarter-final foe will be Australian Jordan Thompson, who beat qualifier Nicolas Moreno De Alboran 6-2, 6-2.
Second seed Frances Tiafoe of the United States was pushed by Romanian qualifier Radu Albot before emerging with a 7-6, 7-6 victory. Tiafoe squandered a 3-0 lead in the first set, and Albot couldn't hold onto a 5-2 lead in the second set, squandering a set point while Tiafoe was serving at 5-6.
Italy's Flavio Cobolli posted a 6-4, 6-7, 6-2 victory over qualifier Zachary Svajda.
