(CNN) -- Tiger Woods faces two challengers as he bids to retain the world's No. 1 golf ranking for a 271st week at the WGC-Bridgestone Invitational starting in Ohio on Thursday.
The 34-year-old has been at the summit for an unprecedented 612 weeks in total, but once again faces the prospect of losing top spot to fellow American Phil Mickelson -- and a new contender, world No. 3 Lee Westwood.
While second-ranked Mickelson has failed to take advantage of Woods' struggles in recent weeks, finishing only tied for 48th at the British Open, Westwood has racked up the points after placing second at his home major last month.
The Englishman can go to No. 1 if he wins at Firestone and Woods is outside the top two -- or if he is second, the defending champion is 10th or lower and Mickelson does not win.
Mickelson can finally surpass Woods if he wins, or if he finishes in the top four and his compatriot is outside the top 37.
"Am I conscious of the pack closing in? Yes, because every tournament you guys remind me," Woods told reporters on Wednesday ahead of his bid for an eighth title in the event.
"You play. How I got here was playing golf tournaments and winning golf tournaments."
Woods will partner Westwood for the first two rounds in Akron, knowing that his rival has finished second in two majors this year and was runner-up at Firestone behind Vijay Singh in 2008.
"I've always enjoyed playing with Westy, he's a great guy. We've been going at it for a long time," he said of the 37-year-old, who he also played with at the U.S. Open in June.
"What he's done over the last couple of years -- I always knew he had that talent and level of play, now he is showing it. Almost every big event we always seem to get paired together. We're going to have a good time."
Woods has dropped out of the U.S. team's automatic qualification positions for October's Ryder Cup after a run of poor results, tying for 23rd at the British Open at St. Andrews last month as he struggled with his putting.
"It has been off all year. I've three-putted quite a few times, which I don't do normally, so I've had to go back to basics and practice a bit more," he said.
"I haven't worked on it as much as I should have the last couple of years, so I've had to go back to that."
Masters champion Mickelson, whose only victory at Firestone came at the World Series of Golf in 1996, will partner young Northern Irishman Rory McIlroy in the first two rounds.
U.S..Open champion Graeme McDowell will play with three-time major winner Padraig Harrington, who returned to form with second place at last weekend's Irish Open.