The Englishman raced to the turn in 31 to move onto nine under par overall and close within a shot of overnight pacesetters Branden Grace of South Africa, Austrian Markus Brier and countryman Miles Tunnicliff.
Scotland-born Dinwiddie was level with another Englishman Ross McGowan, 2010 Open Champion Louis Oosthuizen and Jaco Van Zyl, who were among the later starters.
Local favourite Thomas Aiken, three under in as many holes, and Spaniard Manuel Quiros, one under after completing the first, also began in style to sit a further stroke back on eight under.
The best clubhouse score of the day belonged to another man from England, Matthew Nixon, and Australian Daniel Gaunt, who both carded 68s.
|T4||VAN ZYL, Jaco||RSA||18||-15|