CHICAGO - Two runner-up finishes in the US PGA Tour's FedEx Cup playoffs have only whetted Jordan Spieth's appetite for a victory at this week's BMW Championship.

The third leg of the playoffs tees off on Thursday at Conway Farms near Chicago. His two second-place finishes in the first two legs of the four-tournament series have Spieth atop the playoff standings -- with the series winner after the concluding Tour Championship lifting the FedEx Cup and its $10 million bonus. "Very excited," Spieth said on Wednesday. "I'm in a great position, looking to obviously stay in the number one spot. The FedEx Cup is a tremendous accomplishment, something I'm certainly striving for."

At 24, Spieth already has the 2015 FedEx Cup on his resume, along with three major titles -- the most recent at this year's British Open. His friend and fellow 24-year-old Justin Thomas, who won his first major at the PGA Championship in August, beat Spieth to the Dell Championship title in Massachusets two weeks ago and comes into Conway Farms second in the standings, 27 points behind Spieth.

World number one Dustin Johnson who beat Spieth in a playoff in the Northern Trust, the opening playoff event in August, admitted that tussling with the likes of Spieth and Thomas, not to mention 25-year-old Hideki Matsuyama and 22-year-old Spaniard Jon Rahm, can have him feeling all of his 33 years. "I wouldn't say I feel old, but yeah, I'm the oldest, for sure," said Johnson, currently third in the Cup standings ahead of Matsuyama and Rahm. "They're really good players at a very early age," Johnson said. "I like seeing it. The game needs it, and it pushes me to keep working harder. So I like it."

Any player in the top five in the standings can claim the bonus with a victory in the Tour Championship -- and only the top 30 players will advance to the season finale in Atlanta. "I've won a major," 2016 US Open champion Johnson said. "I haven't won the FedEx Cup. To me, they pretty much go hand-in-hand. It's a big tournament where we get paid very, very well if we win. So it means a lot."

This week's tournament sees defending FedEx Cup champion Rory McIlroy fighting for his playoff life. The Northern Ireland star, desperately trying to salvage a season disrupted by injury, missed the cut at TPC Boston and is 51st in the standings. Reigning Masters champion Sergio Garcia, currently 34th in the standings, needs an 18th-placed finish to advance, while a win would boost him into the top five in the standings.

DAY REPLACES LONGTIME MENTOR SWATTON AS CADDIE: Australia's Jason Day became the latest high-profile golfer to split with his caddie, but said his long association with Col Swatton is far from over.

On the eve of the BMW Championship at Conway Farms near Chicago, Day said Swatton had been a "bit shocked" at the news, after carrying the 29-year-old Australian's bag for all of his career. "Unfortunately it just didn't work out," Day said. "I'm trying to find my footing here. I had to do it at some point."

Day's dominance from late 2015 through the first half of 2016 saw him capture a first career major and rise to number one in the world. But a lackluster 2017 campaign has seen him slide to ninth in the world. Day said Swatton would remain as his coach, but his friend, Luke Reardon, will be on the bag this week.

"He's not out of my world one bit," Day said of Swatton. "I'm hoping things move forward with us like they always have ... because we both love each other like family."

In June, five-time major winner Phil Mickelson split with caddie Jim "Bones" Mackay after 25 years. And last month former world number one Rory McIlroy parted with his longtime caddie J.P. Fitzgerald.