By Mark Lamport-Stokes
(Reuters) - The return from injury by Rory McIlroy to defend his title at this week's PGA Championship at Whistling Straits gives the year's final major extra spice as Jordan Spieth aims to carve out another slice of golf history.
World number one McIlroy and second-ranked 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.
Much of the focus at Whistling Straits will be on the remarkable Spieth, who will be seeking 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 U.S. Open in June, Spieth fell agonizingly 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.
"I recognize there's only been a couple times that people have won three majors in a year, and that would be just such special company," Spieth said while preparing for Thursday's opening round at Whistling Straits in Kohler, Wisconsin.
"But just like at the (British) Open Championship, when I get there, it's just going to be about that tournament, and that's all that will be on my mind ... not thinking about three in a row (or) grand slam.
"It will take a little bit off my shoulders, and it will be very easy for me to just focus on it as a tournament."
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 U.S. 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 soccer with friends on July 4.
However, McIlroy has been diligent with his recovery and his arrival at Whistling Straits on Saturday to play 18 holes of practice at the PGA Championship venue, following by another 18 on Sunday, was warmly welcomed by his peers.
"It's huge," said English world number six Justin Rose. "Any time a tournament misses its defending champion, it's a shame.
"He had such an amazing spell this time last year," Rose added, referring to McIlroy's wins in the last two majors of 2014 - the British Open and PGA Championship.
"I think it's fantastic for golf that he's back, back playing. Obviously, he's in a great pairing next week, him and Jordan and Zach (Johnson)."
Traditionally, the winners of the season's first three majors have always been drawn together and McIlroy, Spieth and British Open champion Johnson are set to start on the first tee at 2:20 p.m. ET (1820 GMT) in Thursday's opening round.
The PGA Championship has often been the most unpredictable of the four majors, in part because it attracts the strongest field, and this week is no different with 97 of the world's top 100 players entered.
Among other early favorites are third-ranked American Bubba Watson, who was beaten by Germany's Martin Kaymer in a playoff for the 2010 PGA Championship at Whistling Straits, fifth-ranked Australian Jason Day and American Rickie Fowler (seventh).
The ability to hit greens in regulation will be a major factor on a par-72 layout measuring 7,514 yards, marking Americans Jim Furyk and Johnson, Swede Henrik Stenson and Australia's Adam Scott among those to watch.
Host venue for the PGA Championship in 2004 and 2010, Whistling Straits is modeled on an Irish seaside links and will once again provide a stiff test with its soaring sand dunes, fast-running fescue fairways and deep pot bunkers.
Eight of the holes hug the Lake Michigan shoreline and the heavily bunkered layout, which includes three par-four holes in excess of 500 yards, is among the longest in major championship history.
"It's a ball striker's golf course," said Spieth. "Tee to green, you just have to be very special. Greens in regulation might be the most important stat next week.
"Around the greens, they're not too tricky to putt. They're subtle. There's not crazy ridges.
"It's a phenomenal golf course."
(Reporting by Mark Lamport-Stokes in Akron, Ohio; Editing by Larry Fine)