Watanuki jumped out to a 4-1 lead in the opening set's tiebreaker, but Auger-Aliassime stormed back and eventually held leads of 5-4 and 9-8. But Watanuki held firm, and on his fifth set point, he was finally able to get past the Canadian.
Australian Jordan Thompson (29) was also able to oust a ranked opponent, beating number seed Adrian Mannarino (35) of France 7-5, 6-4. Fellow Frenchman Gregoire Barrere (29) also fell, losing 2-6, 6-0, 6-3 to ninth-seeded Daniel Evans (33) of Great Britain.
Other Wednesday winners included Dutch number 12 seed Tallon Griekspoor, Americans Christopher Eubanks (27) and JJ Wolf (24), Frenchmen Ugo Humbert (25) and Gael Monfils (36) and Great Britain's Andy Murray (36).
In an all-American showdown, top seed Taylor Fritz (25) saw off qualifier Zachary Svajda (20) in the final match of the night.
Generali Open
Alex Molcan (25) of Slovakia rallied to post a 4-6, 7-5, 7-6 victory, stunning fifth seed Sebastian Ofner (27) in his home country in the second round at Kitzbuhel, Austria.
Molcan had just six aces compared to Ofner's 10, but the former saved nine of 14 break points and overcame a 2-0 deficit in the third-set tiebreaker to advance to the quarter finals.
Molcan will next face Argentina's Sebastian Baez (22), who also pulled off an upset on Wednesday, beating number six seed Roberto Carballes Baena (30) of Spain 6-1, 6-2.
France's Arthur Rinderknech (28) and Colombia's Daniel Elahi Galan (27) also got the best of seeded foes, while Austrian wildcard Dominic Thiem (29), number five seed Laslo Djere (28) of Serbia and Argentina's Tomas Martin Etcheverry (24) and third seed Pedro Cachin (28) won their respective matches.
