Westwood's caddie, Mark Bell, who is the player's manager and part-time caddie, became ill on the 13th hole and had to leave the course. The heat and humidity were too much for Bell, who was replaced by a local caddie.
'It was very hard out there, it felt hotter than yesterday because there was less wind, and Mark was on the brink of collapse with dehydration and exhaustion,' said Westwood.
Retief Goosen, the reigning U.S. Open champion, also posted a 1-under 71 and is one shot back at 4-under-par 212. Stuart Appleby, the second-round leader, struggled to a 2-over 74 and is alone in third at minus-3.
Westwood started poorly on Saturday with three bogeys in his first four holes. He rebounded around the turn with three birdies in a four-hole stretch from the eighth.
'I couldn't quite find my rhythm, but I played some lovely golf after the fourth,' said Westwood, who lost to Ernie Els in a playoff at this event in 2000. 'Those first four holes are not easy and I was only four behind Stuart then, which is not a lot on this course.'
Westwood lost Bell at 13, but hit a fairway-metal from the rough at the par-5 14th. Westwood's ball landed 7 feet from the hole where he converted the eagle putt to take a two-stroke lead.
Westwood dropped a shot at 15 to cut his lead to one with one round to play. He knows that if he is to visit the winner's circle on Sunday, he will need to remain patient on this extremely difficult layout.
'You can't be aggressive on this course like you used to be. You have to be very patient, not try to force it and don't make silly mistakes,' said Westwood, who got into the field when his good friend Darren Clarke withdrew to be with his sick wife. 'The course is playing very long now and there're some very tricky flags tucked away, which you can't get close to even with a sand-iron.'
Goosen mixed two birdies and two bogeys on the front side, and tallied one birdie on the back. He will be looking for his first title here after losing to Sergio Garcia in a playoff last year.
American Chris DiMarco shot a 3-under 69 on Saturday and is alone in fourth at minus-2. Nick Price managed a 2-over 74 and has sole possession of fifth at 1-under-par 215.
Jim Furyk fired a 6-under 66 and is tied for sixth place with three-time champion Ernie Els, who carded an even-par 72 on Saturday. The duo is knotted at even-par 216.
British Open winner Todd Hamilton (71) is eighth at plus-1, followed by 50-year-old Jay Haas (70), who is through three rounds at 3-over-par 219.
Garcia (74) and Fredrik Jacobson (72) share 10th at plus-9, while Chad Campbell, who posted an even-par 72 on Saturday, is in last place at 11-over- par 227.