Louis Oosthuizen birdied the 18th for a 67 and two-round total of -12 at St Andrews.
The South African shot a 65 yesterday and teed off in the second group this morning, making the turn in 33 with birdies at 5, 6 and 7.
He started the back nine birdie, bogey, birdie, bogey, birdie before steadying the ship with three pars and rolling in a 15-footer for birdie on the final green.
His clubhouse total of -12 is three shots clear of nearest challenger Rory McIlroy, who begins his second round at 1.31pm.
Oosthuizen said after the round: "I've been playing well the whole year and it's just a matter of making putts.
He added: "St Andrews is where it all started and I think it's everyone's dream to win an Open to do it at St Andrews would be great."
Paul Casey and Ricky Barnes are putting together good rounds and sit at -8 while Mark Calcavecchia is in the clubhouse on -7 after signing for a 67.
Find out how Louis Oosthuizen rolls in those birdie putts in the latest issue of Today's Golfer, on sale now.