Ignacio Garrido (66) and Ernie Els (67), a two-time winner this season, share third place at 11-under-par 205.
Immelman entered the round three shots behind overnight leader Darren Clarke and started quickly with a birdie at the par-4 first. After a pair of pars, Immelman notched his next birdie at the par-5 fourth.
Immelman, who won the South African Airways Open earlier this season, converted his third birdie of the day at the par-4 sixth. He made the turn at nine-under for the tournament.
On the back side, Immelman caught fire at the par-4 11th. He rolled in a birdie there and followed with a birdie at the next. Immelman made it three straight when he sank another birdie putt at the par-4 13th.
For the second time in the tournament, Immelman bogeyed the par-4 15th, this time due to a three putt. He responded by again running off three straight birdies. That run began with a lengthy birdie putt at 16 and was capped by a two-putt birdie at the par-5 18th.
'That was pretty much as good as I can play,' Immelman said. 'It was a lot of fun to be able to be out there with Ernie Els and shoot a round like that.'
Immelman and Els, a pair of South Africans, played together Saturday, something that inspired Immelman.
'I remember sitting up at midnight and watching Ernie win two U.S. Opens and then got to play in the British Open Championship last year when he won,' said Immelman. 'This was something I was hoping I could get to one day.'
Els carded back-to-back birdies from the second and followed that with an eagle at the par-5 fourth. However, he bogeyed three straight holes from the seventh.
The reigning British Open champion recovered from there, with consecutive birdies from the 11th and again from the 17th for a 5-under round of 67. Playing with Immelman, Els realized how well the youngster is playing.
'I know Trevor as well as anybody,' said Els. 'He's going to be our next star and he's going to show it. If I want to win tomorrow, I've probably got to go through him.'
Fasth recovered from a late double-bogey that cost him a share of the lead. After two bogeys and three birdies on the front side, Fasth birdied 10 and 12. He then double-bogeyed the par-4 15th but recovered to tally birdies on each of the final three holes to move back into second place.
'Obviously it is much better for tomorrow after the finish,' Fasth said. 'I did not really get the scoring going today as I would have liked to. That double-bogey could have killed me but I chose to really fight back. Tomorrow we just have to go out and give it our best. Two shots can be a lot or nothing.'
Paul Casey also carded an 8-under 64 Saturday to move into a tie for fifth. Mathias Gronberg and Gordon Brand, Jr. share fifth with Casey at 10-under-par 206.
Clarke, who owned a one-stroke lead heading into the round, struggled all day. He dropped six shots off the lead at one point after a front nine that included two double-bogeys and three birdies. After a bogey at No. 10, Clarke moved back to even-par for his round with birdies at 12 and 17.