Westwood led by 11 strokes at the halfway stage after going bogey-free on the opening two days but a 1-over 73 in the third round allowed Schwartzel to cut the deficit.
Schwartzel holed four straight birdies from the second hole and was within three shots of the lead heading to the last but a bogey gave Westwood some breathing space.
“I didn’t expect to be four shots back at the start of today’s round,” Schwartzel said. “I knew I had to shoot a low score. Lee didn’t play particularly well. I’ve seen him play much better golf.
“I feel I still need to shoot a 64 or 65 on the last day to get a chance. At least there’s a light at the end of the tunnel now. Four shots is not much out there.”
Schwartzel came from four shots back to win his first major at The Masters in April.
“I’ve won about two or three tournaments where I was four shots behind,” he said. “I don’t know what the secret is. I just play my own game when I’m out there.”
Westwood shot a career-low 60 in the first round and followed that with a 64. He had four bogeys on Saturday as his form dipped.
“After a 60 and 64, you have to be a bit realistic unfortunately,” Westwood said. “A lot of people are going to think you’re going to shoot 60, 62 and 64 every day but I’m afraid golf is not like that. Every day you have to reset your goals and your sights.
“A four-shot lead into the last round is a good position to be in.”