Roger Federer and Rafael Nadal's bid to end 2017 as the world number one is expected to go right to the wire as the tennis season gets set to resume in earnest at the Rogers Cup in Canada. The pair are the top two seeds for the event in Montreal, with Andy Murray's status atop of the ATP rankings hanging by a thread.
In the absence of Murray, Nadal is best placed to return to number one spot for the first time since June 2014. Should the Spaniard reach the semi-finals in Canada he will dethrone the Briton, while taking the title will see him claim a record 32nd Masters 1000 title and move ahead of Novak Djokovic.
Federer, fresh from winning his 19th grand slam title at Wimbledon, is unable to return to the summit of the men's game - a position he has not occupied since October 2012 - next week. But he can lay the platform for an assault on top spot during the remainder of the North American hard court season should he win the event for the first time in 11 years.
And though the ranking could change hands next week, former American player Leif Shiras believes the battle to be the year-end number one could go down to the final tournament of the season; the ATP World Tour Finals in London in November.
"Roger and Rafa are the players with momentum and confidence, but Rafa must be feeling pretty bullish because he loves to compete and wants to prove that he's still the guy," Shiras told Sky Sports.
"I get the sense that he's playing a little more aggressively, he's trying to commit to being up on the baseline and although Wimbledon didn't quite work out for him, we'll have to see about the hard courts.
"I think it will be a photo finish at the end of the year with Roger and Rafa's noses poised over the line in the ATP World Tour Finals championship match at London's O2. I think it will be great for tennis and might be something that we may never see again."
Federer and Nadal have shared the four Masters 1000 titles already played this season, with the Swiss prevailing in Indian Wells and Miami before his long-time rival won on the clay in Monte-Carlo and Madrid, ahead of claiming his 10th French Open crown. The conditions in Canada are expected to be more to Nadal's liking but Shiras is not ready to totally discount Federer, who turns 36 next week, from winning the title.
"It is the Rogers Cup after all," he added. "The conditions are slower in Montreal and Federer's favourite North American event next to the US Open and Indian Wells is Cincinnati where it's quicker. Then again, I'm not going to discount Roger."