KOHLER - Rory McIlroy's return was the last thing on Jordan Spieth's mind as the PGA Championship teed off on Thursday with the young US star set to go head to head with the world number one.

It's a date that has been delayed by the left ankle injury that kept Northern Ireland's McIlroy from defending his British Open title at St. Andrews -- where Masters and US Open champion Spieth narrowly failed to capture the third leg of a calendar Grand Slam. Spieth can still become just the third player -- along with Ben Hogan in 1953 and Tiger Woods in 2000 -- to win three majors in the same season.

He could also wrest the world number one ranking from McIlroy, who was to tee off alongside Spieth and British Open winner Zach Johnson at 1:20pm (18:20) at Whistling Straits, the 7,501-yard, par-72 links-style course laid down on the shores of Lake Michigan. "I think that's just what you guys want to see," Spieth said of a showdown with McIlroy. "I think he and I just want to go out there and try and win the tournament. We have to beat each other in order to do that, along with ... 155 other guys. "Hopefully, we can all get into contention, and it will certainly be exciting."

McIlroy calls Spieth's achievements this year -- which include two PGA Tour wins in addition to his first two major titles -- nothing short of inspiring. McIlroy, winner of the British Open and PGA Championship in a stellar 2014 campaign, has three wins this season but has yet to claim a major in 2015. "If you were to go by this year, you would have to say Jordan," McIlroy said when asked to name the best golfer in the world. "If you go over the last two years, I would say it's probably a toss-up between Jordan and myself ... it's all a matter of opinion." Pressed to offer his own opinion, McIlroy responded: "I'll tell you at the end of the week." While McIlroy insisted four practice rounds in Portugal last week had proved to him his ankle is "100 percent," that is sure to be tested on the rolling terrain of Whistling Straits.

The Pete Dye-designed course produced PGA Championship playoff victories for Vijay Singh in 2004 and Martin Kaymer in 2010. "I thought it was going to be a bomber's paradise," Spieth said of the course he first played last week. "The fairways are very narrow here and the rough is thick enough to where it affects these approach shots where you really, really do need to be in the fairway." Then there are the bunkers -- more than a thousand since every patch of sand at Whistling Straits is so designated. "The bunkers can be a little inconsistent," Spieth said. You can catch a good lie and you can catch a lie where it's tough to advance it without hitting it thin. So I think there's more of a premium on hitting a fairway than there is on just bombing it out there and playing in from there."

Among the early starters Thursday, South African Rory Sabbatini was off to a hot start with three birdies in the first four holes. Tiger Woods, whose 14 major titles include four PGA Championships, was due to tee off at the 10th hole at 8:15 am (13:15 gmt) with 2011 winner Keegan Bradley and Kaymer.