A bogey on the last hole saw Sergio Garcia card a 71 to hold a two-shot lead after the third round of the European Tour's Nedbank Golf Challenge following a day of high scores at the Gary Player Country Club.
Garcia managed three birdies and two bogeys, including the one on the 18th for the second round in a row, as the players battled with challenging pin placements and a swirling breeze.
Only 28 of the remaining 71-man field managed under-par scores on Saturday, but Garcia shot a 10th successive sub-par round. The Spaniard, who won the title in 2001 and 2003, is at 10-under for the tournament and leads from home favourite Louis Oosthuizen, who shot an even-par 72.
The South African went down the 18th fairway four shots back but his birdie and Garcia's dropped shot, after a wayward approach to the green that saw him put his iron into the gallery, halved the deficit.
"It [the course] was playing difficult, so I felt like I played quite solid. I gave myself some good chances," Garcia told reporters. "I probably shot the highest I could shoot today. Leading by two, it's a good position to be in.
"It is going to be tough [on Sunday], but it's good to come with some good momentum, to be playing nicely and in the lead."
"I always look forward to being in contention, even more so when it is a golf course like this where I have won before," two-time champion Westwood told reporters.
The lowest round of the day came from England's Ross Fisher (67) as he overcame a calamitous finish on Friday in which he dropped eight shots in the final four holes.
Despite that collapse he is in contention and goes into the final round on 6-under.
"Golf can bite you at any moment,' a rueful Fisher told reporters. "You can think about it, you can cry about it or you can just get over it and take the positives from your 14 holes.
"That's how I looked at it today, came out with a positive mindset and pleased to shoot 67."