Lee Westwood carded a course-record 62 to storm into a seven-shot lead in the Nedbank Golf Challenge at the Sun City resort in South Africa.
The Worksop golfer's magnificent round featured 10 birdies and not a single bogey and left the world number three well on course to retain his title and claim a first prize of almost £800,000.
Second-round leader Graeme McDowell shared second spot with Sweden's Robert Karlsson on nine under, with American Jason Dufner and Germany's Martin Kaymer tied for fourth a shot further back.
Birdies came in bursts for Westwood - first between two and four, another run of three between nine and 11 and most spectacularly of all a streak of four between the 14th and 17th.
Westwood found the edge of the green with his approach to the 18th as he chased a fifth birdie in a row, but left his putt just short.
McDowell trailed by just five going to the 18th but made a mess of the final hole, twice going into bunkers before making a double bogey which left him with a 70 for the day, two under par.
Home favourite Charl Schwartzel recorded the second-best round of the day with a four-under 68 which left him in a share of sixth with Kim Kyung-tae.
World number one Luke Donald was in eighth after a 70, while Denmark's Anders Hansen was the highest scorer of the day, shooting a five-over 77 in an up-and-down round which even featured an eagle three at the par-five ninth.