Roger Federer will consider his fitness before continuing his pursuit of history and a return to world number one at the Cincinnati Masters this week. The 19-time grand slam champion could become the oldest man ever to sit atop of the ATP standings if he wins the latest Masters 1000 event of the season but his run to the final of the Rogers Cup has potentially jeopardised his plans.

Defeat to Alexander Zverev in Montreal denied Federer the chance to go above Rafael Nadal and gives the Spaniard the upper-hand in the race to replace Andy Murray at the summit of men's tennis. Federer must reach the semi-finals to stand any chance of denying the 10-time French Open champion a return to number one, with anything less ensuring Nadal goes back to the top for the first time in three years.

Neither player has any points to defend this week and with last year's beaten finalist Murray having withdrawn due to an ongoing hip problem, either one of Federer or Nadal will be the new number one heading into the US Open when the rankings are recalculated next week. But after his two-set loss to Zverev, Federer admits the physical after-affects from playing five matches in as many days could thwart his campaign again.

"I'll travel to Cincinnati, then take a decision in the next couple of days and see how I feel after five days of playing, if I'm ready to play in Cincy next week or not," he told reporters.

"You know, felt all right all week. Had a bit of muscle pain, aches and pains here and there, just because it's back on the match courts, on the hard courts. After vacation and practice, it's always a bit of a shock for the body. We'll have to wait and see now how I feel in the next couple of days."

Both Federer and Nadal have first round byes in Cincinnati. Nadal is the top seed and could face either Richard Gasquet and John Patrick-Smith in the last 32 while Federer's opening opponent will come from Diego Schwartzman and Karen Khachanov's opening round clash.

Should Federer play his second round match then he will go ahead of Murray in the rankings, ensuring that he starts his US Open campaign at Flushing Meadows inside the top two in the world again. New York could yet be the scene of Federer's 20th major title, and third of the year, a feat the 36-year-old feels would be "insane".

"Oh, it would be totally incredible," the five-time previous winner added. "From right now on till the US Open, I got to think what's my best opportunity to feel well there. There's going to be a ball change, as well, which creates a lot of different plays, if you like.

"I've never played in the new stadium as such because I missed last year's edition. Yeah, like I said, next couple days are important for me to take the decision on Cincinnati.

"Then looking ahead to the US Open, obviously I want to be in the best possible shape. Winning my third of the year, my 20th Grand Slam, would be completely insane. I just hope I'm going to be 100% ready when the moment arises."

Roger Federer
Federer may have lost in the Rogers Cup final but his pursuit of world number one is not over. Getty Images