Rafael Nadal of Spain hits a shot against Borna Coric of Croatia (not pictured) during the Rogers Cup tennis tournament at Uniprix Stadium in Montreal, Quebec, Canada, Aug 9, 2017. [Photo/Agencies]

Nadal has spent 141 weeks at No 1 since first ascending there after reaching the Western & Southern semifinals in 2008.

"It's been tough to get back to No 1," he said. "I'm happy to have the chance to be in that position."American Sam Querry became the first seeded player to advance. The 15th seed cruised into the second round with a 6-3, 6-0 win over wild-card Stefan Kozlov.

In women's first-round action, 2016 Olympic singles gold medalist Monica Puig lost to fellow qualifier Taylor Townsend 6-3, 3-6, 6-2.

Other's men's matches saw wild card Frances Tiafoe beat qualifier Maximilian Marterer 6-3, 7-6 (2) and qualifier Joao Sousa defeated Kyle Edmund 6-3, 2-6, 6-3.

Richard Gasquet and wild card Tommy Paul advanced in straight sets. Gasquet eased past qualifier John-Patrick Smith 6-4, 6-4, and Paul beat fellow American Donald Young 6-4, 7-6 (4). Ivo Karlovic ousted Jiri Vesely 6-3, 3-6, 7-5.

Also advancing in straight sets were Fabio Fognini, a 7-6 (5), 6-4 winner over Daniil Medvedev; qualifier Mitchell Krueger, 6-2, 6-1 over Benoit Paire; and Feliciano Lopez, 7-6 (5), 6-1 over Hyeon Chung.

In other women's matches, Roberta Vinci knocked out Time Babos 7-5, 7-5 and Camile Giorgi ousted Katerina Siniakova 6-2, 6-2.

Also, 14th-seeded Petra Kvitova rallied past Anett Kontaveit 1-6, 7-6 (2), 6-3; Beatriz Haddad Maia topped Lauren Davis 6-3, 6-2; and Lesia Tsurenko defeated Anastasia Pavlyuchenkova 5-7, 7-6 (6), 6-0.

Kristina Mladenovic, seeded 13th, became the first seeded player to lose, beaten by Daria Gavrilova 6-0, 7-6 (6).

AP