Both Tiger Woods and world No. 1 Rory McIlroy failed to make the cut.
Westwood, who carded a two-under-par 70 at Kingsbarns to lie four off the lead, would usurp Tiger Woods as world number one with a top-two finish.
The 37-year-old Ryder Cup star needs to finish in the top two to replace Tiger Woods as world number one but ended 10 shots off the pace at two under.
Westwood will not play for a month to recover from injury - a move that was set to mean he would move ahead of Tiger Woods as world number one.
He said he uses standard techniques employed by tiger experts around the world.
Richard Parker immediately evoked for me a real-life tiger in the negotiating world.
World number Tiger Woods had a superb day and fired nine birdies to reach 11 under overall when bad light stopped play.
Poulter now heads to Shanghai for the WGC-HSBC Champions and is confident of carrying over his form against Tiger Woods and a world-class field.
The No. 1 and No. 2-ranked players in the world, Tiger Woods and Rory McIlroy, had entourages that included girlfriends with some pedigree of their own.
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The Barclays organizers have confirmed that Tiger Woods and new world number one Rory McIlroy will be paired together for the first two rounds at Bethpage.
Deposed world number one Tiger Woods is five off the lead after a difficult 72.
World number one Tiger Woods played superbly in the testing winds and almost holed his drive to the par-four 18th.
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World number one Tiger Woods, battling a dose of flu, returned a two-under 69 as he chases a third successive win.
He is still seeking his first victory this season, having fallen to No. 2 in the world rankings behind Tiger Woods.
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The United States, meanwhile, were without injured world number one Tiger Woods as well as having a total of six debutants in their 12-man team.
World number one Tiger Woods finished in a tie for 23rd at three-under after a final round 72, with arch-rival Phil Mickelson tied for 48th at one-over.
Meanwhile, US captain Corey Pavin put world number one Tiger Woods alongside Hunter Mahan while Steve Stricker, who won all four matches alongside Woods in the Presidents Cup a year ago, joined Zach Johnson.
Yet Oosthuizen left Paul Casey, Lee Westwood and the world number one Tiger Woods in his wake, with his final round of one-under-par 71 giving him a 16-under total of 272 and his only major title so far.
World number one Tiger Woods, who won his third major and the first of his three US Open titles by a record 15 shots when it was last held at Pebble Beach in 2000, began with eight straight pars and found the first 11 greens in regulation as he demonstrated a more controlled long game than in recent times.
This week's tournament is the first since Englishman Westwood took Tiger Woods' title as the world's best-ranked player on Sunday.
Former world number one golfer Tiger Woods and US Olympic skiing champion Lindsey Vonn have announced that they are dating.
World No.1 Tiger Woods produced his second 68 of the weekend to finish tied for ninth on 11-under in the first strokeplay event of his comeback.
As part of the deal, McIlroy changed his golf clubs to play with Nike branded equipment, which is also endorsed by his great rival and World No.2 Tiger Woods.
Early starters prospered in good scoring conditions on the 7, 468-yard course in New York State, including world No. 3 Tiger Woods and top-ranked new PGA champion Rory McIlroy, who were paired together with former Masters champion Zach Johnson.
The top two golfers in the world, Rory McIlroy and Tiger Woods, have both spoken in favor of a ban.
CNN: Golf's major tours split over proposed belly putter ban
The top two golfers in the world, Rory McIlroy and Tiger Woods, have both spoke in favor of a ban.
CNN: Golf's major tours split over proposed belly putter ban
When we looked at the highest-paid athletes in the world three years ago, Tiger Woods narrowly topped the list ahead of German Formula One legend Michael Schumacher.
Westwood, needing to finish in the first two to dethrone Tiger Woods at the top of the world rankings, was 10 shots back after the second day.
Justin Bieber is already king of the music world, social media and the Tiger Beat crowd.
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