Ernie Els also fired a 63 on Saturday and vaulted into a tie for fourth. The two-time U.S. Open champion is joined at 13-under par by first-round co-leader Paul McGinley (66), overnight co-leader Roger Wessels (69) and last year's runner-up, Justin Rose (66).
Maritz flew out of the gates in the third round with birdies at the 1st and 3rd holes. He ran home a 20-footer at the 4th and added a three-foot birdie at No. 5 to reach 12-under for the tournament.
The 24-year-old South African continued his torrid pace at the par-3 6th when his eight-foot fringe putt found the bottom of the cup. At No. 7, Maritz spun his approach to a foot, setting up the tap-in birdie, his fifth in a row.
Maritz parred the next two but came back to his birdie form with a 20-footer at No. 10. He made it back-to-back birdies with a 10-footer at the 11th. He missed some putts over the next four holes but saved par and added a birdie at the par-5 16th to pad his lead to two shots.
Maritz earned three top-20s in five European Tour starts last year and now he fully realizes that he can gain a two-year exemption if he can hang on Sunday.
'In about 24 hours my life can change,' said Maritz, who tied for ninth last week at the South African Open. 'Winning was definitely my goal coming into this week and hopefully Ill be sitting here tomorrow with good news.'
Maritz's 63, his lowest round as a professional, matched Els' 9-under from earlier in the round. Maritz felt good about that considering he modelled his swing after the Big Easy.
'Ive always wanted to swing like Ernie since I was a young boy or 12 or 13,' he said. 'If my swing can be half as good as his swing then Ill be happy.
'Foster posted a flawless 7-under 65 on Saturday. He picked up three birdies on his second nine to earn his spot in the final pairing on Sunday with Maritz.
The 26-year-old won the 2000 European Challenge Tour ranking title and after taking a fourth at the Omega Hong Kong Open in his second start on the European Tour at the end of November, he believes he can win early on the big tour.
'I was hoping that winning the Challenge Tour would give me confidence and self-belief. It certainly gives you a lot of experience for situations like this,' said Foster, who has yet to drop a stroke in this tournament. 'The standard has improved and the cut marks are lower. It gives you good experience of travel and four round tournaments.'
Grewal broke into red figures early with three birdies in his first five holes, but he could only manage two birdies the rest of the way. He carded a 68 in the third round after he bogeyed the 13th.
Els rebounded from a 72 on Friday with his 63. He carded nine birdies and two front-nine bogeys but finished with an eagle-3 at the closing hole to get within four of the lead.
'At least Ive got a shot at it tomorrow,' he said. 'I felt good on the range this morning. My swing felt nice and tight at the top and I felt I could hit the ball solidly. Its good that I shot my lowest round for quite a while.'
Mark Mouland is alone in eighth at 12-under par after a third-round 67.
Reigning U.S. Open champion Retief Goosen still has yet to make a move up the leaderboard. He shot a 2-under 70 on Saturday and is tied for ninth at 11-under with Christian Cevaer, who shot a 68 in the third round.
Shot of the day honors go to James Kingston. He hooked his four-iron into the trees at the par-three 15th but his ball hit a branch, landed on the green and fell into the hole for an ace.
Full-Field Scores from the Dunhill Championship