ANCASTER, Ontario - A couple of key changes have stacked the odds against Dustin Johnson as he tries to defend his RBC Canadian Open title.

The No. 2 player in the world pulled away from the field after a nearly two-hour rain delay to win his first Canadian Open last year, firing a 6-under 66 in last year's final round to finish at 23 under. But this year the only Canadian stop on the PGA Tour has moved from Glen Abbey Golf Club - a course Johnson is familiar with - to Hamilton Golf and Country Club.

More importantly, the tournament has moved up in the golf calendar, making it a lead-in event to the U.S. Open and drawing a significantly stronger group of competitors.

''I mean, it's a really good field. It's a golf course I haven't played. That definitely adds a little bit more difficulty to it,'' Johnson said Wednesday. ''I don't know the golf course as well as I know Glen Abbey, where I played a lot of Opens.''

Johnson is joined by top-ranked Brooks Koepka, fresh off of his PGA Championship win, No. 4 Rory McIlroy and No. 6 Justin Thomas, who was added to the field on Friday.

Historically, the Canadian Open was held in September, but starting in 2007 it was played in late July, the prime golf season. Unfortunately, it was also the week after the British Open, causing many of the PGA Tour's top players to miss the tournament as they recovered from the challenging major.

Now in early June, many golfers are using the Canadian event as a tune-up for the U.S. Open.

''This isn't just a preparation week. This is a very prestigious tournament, one of the oldest tournaments in the world that I would dearly love to be able to add my name to,'' said McIlroy, who has never played a competitive tournament in Canada. ''I'm fully focused on this week, but knowing that if I play well here, this week, and have good control of my ball and my distance control, that will serve me well going into next week.''

One problem for Johnson, Koepka and McIlroy is that none of them had a chance to play a full practice round. With off-site sponsor obligations and a pro-am tournament on Monday, they played only half a round as part of a pro-am on Wednesday because of rain.

''I like the front nine. It's the only nine I've seen, but yeah, I like the golf course,'' said Johnson, whose brother Austin serves as his caddie. ''(Austin) went out Tuesday and kind of looked at it. He's good enough now where he can get a pretty good beat on the course.''

That lack of familiarity may give the 26-player Canadian contingent a brief edge, at least for the first round.

''I think it's a huge advantage. There's an added comfort level, just knowing the course and knowing what to expect,'' said Corey Conners, who played a practice round in May. ''I'm really happy that I got the chance to play a few weeks ago. I got a plan together and to be familiar with the terrain, know what to expect and just definitely an added comfort level for the Canadians.''

A total of 26 Canadians will be competing for the national championship. No Canadian has won the event since Pat Fletcher in 1954.