Roger Federer withdraws from Cincinnati; Rafael Nadal to become new No. 1

Mason: Wimbledon champion Roger Federer withdrew Monday from the Western & Southern Open, 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.

Federer's withdrawal means Rafael Nadal 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.

In early first-round men's play Monday, 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.

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.