Basel, Aug 15 (IANS): Swiss tennis legend Roger Federer has pulled out of the Western & Southern Open in Cincinnati, Ohio, due to a back problem, making Spain's Rafael Nadal the World No.1 when the rankings come out on August 21 regardless of how far he advances at the event.

"I am very sorry to pull out of the Western & Southern Open as I always enjoy playing there," Federer said in a statement on Monday.

Britain's Andy Murray, the current World No.1, also decided to skip the Masters 1000 tournament in Cincinnati, reports Efe.

On Sunday, German rising star Alexander Zverev handed Federer a rare loss in the 2017 season, rolling to a 6-3, 6-4 win over the Swiss star in the final of the Rogers Cup in Montreal.

"Cincinnati has some of the best fans in the world and I am sorry I will miss them. Unfortunately, I tweaked my back in Montreal and I need to rest this week," Federer, a seven-time champion at the Western & Southern Open, said.

The 36-year-old Federer, winner of a record 19 Grand Slam titles, has turned back the clock in 2017 with major titles at both the Australian Open and Wimbledon.

Nadal, who has 15 Grand Slam singles titles under his belt and won his record 10th French Open title on June 11, will be the top-ranked player in the world for the first time since July 2014.

The 31-year-old Nadal will be the top seed in Cincinnati, which started on Sunday and is one of the key tune-up events for the US Open, which begins on August 28.