MASON, Ohio >> Wimbledon champion Roger Federer withdrew from the Western & Southern Open today, which he has won seven times, citing a back injury.

Tournament officials announced Federer’s withdrawal on the first full day of matches at the event in Cincinnati.

The 19-time major champion said in a statement that he “tweaked” his back last week at the Rogers Cup in Montreal, where he lost Sunday to Alexander Zverev in the final.

The Swiss star, No. 3 in the ATP Tour rankings, becomes the fifth of the top six players to skip Cincinnati because of injuries. He joins top-ranked Andy Murray, No. 4 Stan Wawrinka, fifth-ranked Novak Djokovic and sixth-ranked Marin Cilic, the defending champion. Only second-seeded Rafael Nadal, the tournament’s top-seeded player, remains in the draw.

“It’s just coincidence,” said Nadal, 31, pointing out that all of the missing players except Cilic are 30 or older. “We’re not 20 years old any more. We’re not playing all the weeks. It’s part of our sport. I’ve been in their position lots of times. I’ve missed more events than the other players. It’s part of the game. I wish them all a speedy recovery. We need them in the game. I hope they get back soon.”

Federer’s withdrawal means Nadal, who lost in the third round at Montreal, will return to No. 1 when new ATP rankings are released on Aug. 21. The Spaniard will be No. 1 for the first time since July 6, 2014.

Nadal has spent 141 weeks as the men’s No. 1 player since first ascending to the top spot 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.”

In early first-round men’s play today, Richard Gasquet and wild card Tommy Paul advanced in straight sets. Gasquet eased past qualifier John-Patrick Smith 6-4, 6-4, while Paul beat fellow American Donald Young 6-4, 7-6 (4). Ivo Karlovic needed three sets to knock out Jiri Vesely 6-3, 3-6, 7-5.

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

In women’s action, 14th-seeded Petra Kvitova shook off a first-set loss to beat Anett Kontaveit 1-6, 7-6 (2), 6-3, while Beatriz Haddad Maia advanced with a 6-3, 6-2 win over Lauren Davis. Lesia Tsurenko also crafted a comeback win, defeating Anastasia Pavlyuchenkova 5-7, 7-6 (6), 6-0.

Kristina Mladenovic became the first seeded player to be eliminated. The 13th seed lost to Daria Gavrilova 6-0, 7-6 (6).