(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. 
How can Phil Mickelson become world No. 1?
Has he been able to take advantage of the opportunity so far?
How so?
Who else is in the running?
Anyone else?
Who?
Where is he from?
What event starts on Thursday?
Where is it being held?
What city?
How can Westwood become No. 1?
Who will play together the first two days?
How old is Tiger?
if he wins at Firestone and Woods is outside the top two
 if he wins at Firestone and Woods is outside the top two
No
Mickelson has failed to take advantage of Woods' struggles in recent weeks
he was finishing only tied for 48th at the British Ope
inishing only tied for 48th at the British Ope
Woods
Woods 
Yes
Lee Westwood
Lee Westwood
Lee Westwood
England
 Englishman 
WGC-Bridgestone Invitational
 WGC-Bridgestone Invitational
Ohio
 Ohio 
Akron
 Akron
if he is second, the defending champion is 10th or lower and Mickelson does not win
if he is second, the defending champion is 10th or lower and Mickelson does not win
Mickelson and Woods
Mickelson can finally surpass Woods
34-year-old
34-year-old