Toronto - Top-ranked Rafael Nadal will be working to increase grip atop the rankings at the ATP Toronto Masters, an event which lost an exhausted Andy Murray prior to Monday's start.
Britain's Murray, winner of three matches in four days this week in Washington, withdrew from Washington and Canada on Friday after finishing his latest victory after 3 a.m., breaking down in tears after the match.
But the cumulative effort was too much for the 31-year-old Scotsman, who underwent right hip surgery in January.
"I'm exhausted after playing so much over the past four days, having not competed on the hard courts for 18 months," Murray said.
"I also need to be careful and to listen to my body as I come back from a long-term injury."
The former number one, now ranked 832, was playing the third event in his slow-running comeback after first returning in June on grass in Britain before withdrawing from Wimbledon.
World number two Roger Federer is skipping Canada and will return on August 13 in Cincinnati, where Murray is also expected.
Federer, limiting his schedule to extend his longevity, turns 37 on Wednesday.
Nadal will be making his first appearance on court since a five-set Wimbledon semi-final loss to Novak Djokovic.
With most of the rankings top 10 in Canada, Nadal's path to a potential fourth title in the country after 2005, 2008 and 2013 will be a predictable challenge.
The Spaniard resumed training recently on his home island of Mallorca after spending off-time sailing.
Nadal opens play on Wednesday against either American Jared Donaldson or Frenchman Benoit Paire, fined $16,500 this week for a racquet-smashing outburst in Washington.
The 17-time Grand Slam champion isn't looking ahead at other potential foes such as Swiss three-time Grand Slam winner Stan Wawrinka or Australian Nick Kyrgios.
"I cannot think about Wawrinka, Kyrgios, Joao (Sousa) before playing Benoit or a qualifier," Nadal said.
"I know the first round is going to be a tough battle. But the full draw doesn't distract me from being focused on that first round."
"I always take it match by match. That's what I've done all my life. That's the only way, in my opinion, to have success.
"You need to respect all opponents. In tennis, things change so quickly, so you need to be ready to accept both victories and losses."
Second seed Alexander Zverev will defend his crown after beating a tiring Federer in the 2017 title match at the event, which alternates annually between Toronto and Montreal.
The 21-year-old German has shown he has the game to challenge the elite, winning Masters 1000 trophies in Rome, Montreal and Madrid.
But his weakness so far has been the Grand Slams, where the NextGen talent can boast just one quarter-final from Roland Garros last spring.
With his big-match format suspect, durability has also been a recent problem for Zverev.
He lost at Roland Garros in his third successive five-set match as he played Ernests Gulbis and was tested in a pair of three-setters in Washington.
Zverev will come to Canada having had little chance to rest and regroup, starting with either Spain's David Ferrer or a qualifier.
Third seed Juan Matin del Potro will be hoping to make up for a second-round loss a year ago, taking on either Japan's Kei Nishikori or Dutchman Robin Haase.
Wimbledon runner-up Kevin Anderson takes the fourth seeding, the South African to open against Russian Andrey Rublev or a qualifier.
Wimbledon winner Djokovic starts with South Korea's Chung Hyeon, an Australian Open semi-finalist bothered by injury much of this season.