KOHLER, Wisconsin - Clear skies and barely a breath of wind greeted the early starters at the PGA Championship on Thursday as golf fans prepared for the 'Rory and Jordan' show later in the day at Whistling Straits.
The return from injury by world number one McIlroy to defend his PGA Championship crown this week and Spieth's bid to win a third grand slam title this year have marked out the season's final major as extra special.
McIlroy and Spieth have between them won four of the last five majors played and, aged just 26 and 22 respectively, they are likely to dominate the game at the highest level for some time to come.
Traditionally, the winners of the season's first three majors have always been drawn together and McIlroy, Spieth and British Open champion Zach Johnson are set to start the opening round on the first tee at 2.20am ET (2.20am Friday Singapore time).
"I'm excited to just share a couple days with Rory and Zach," said Masters and US Open champion Spieth. "I played with Zach (at Firestone) last week, he's a good friend of mine.
"We're going to have a good time, and hopefully we can all feed off each other these two days. Hopefully we can all get into contention."
Much of the focus this week will be on the remarkable Spieth as he seeks to join fellow Americans Ben Hogan and Tiger Woods as the only players in the modern era to have won three majors in a single year.
Hogan was the first to complete the hat-trick, in 1953, and Woods followed suit in 2000.
Having claimed the Masters in April and the US Open in June, Spieth fell agonisingly short in his bid to land the first three majors of the season as he tied for fourth in last month's British Open at St. Andrews one shot short of joining a playoff.
Four-times major champion McIlroy will also draw intense scrutiny at Whistling Straits as he returns to competitive golf after an absence of more than seven weeks.
The Northern Irishman, who tied for ninth in the US Open at Chambers Bay in his most recent start, has been out of action since he ruptured a ligament in his left ankle while playing football with friends on July 4.
However, McIlroy has been diligent with his recovery and said earlier this week he fully expected to play well. "I don't see any reason why I can't bring the sort of form that I've shown in practice rounds and on the range to the tee on Thursday afternoon," the Northern Irishman said.
Spieth, McIlroy and Johnson could end up with the tougher half of the draw on Thursday as wind gusts of up to 45kmh have been forecast for later in the day. "If the wind is up, that will be the teeth of the course," said Spieth. "(This) may be a tougher day to get started."