Prayad Marksaeng had a commanding lead until a disastrous triple-bogey on the 17th hole. He birdied 18 to shoot a 3-under 69 and is tied for second place with Marcus Both, who carded his second consecutive 4-under 68. The pair is knotted at minus-8.
Wade Ormsby managed a 1-under 71 on Friday and is alone in fourth place at 6-under-par 138.
Mamat, who shared the lead after Thursday's opening round, struggled on the front nine Friday with two bad drives leading to a pair of bogeys on both front-nine par-5s, the second and seventh holes. He birdied eight to make the turn at 1-over.
'To bogey the par fives is a nightmare for us,' admitted Mamat, a former winner on the Asian Tour, which is co-sanctioning this event along with the European Tour. 'After that, I managed to get back some birdies. It was a good comeback.'
He birdied the 10th, then appeared to be in trouble on 13. Mamat's drive seemed destined for water, but caught a wooden staircase and bounced into a bunker. He made par from there, then birdied No. 14.
At the closing hole, Mamat hit a knock-down 9-iron to six feet. He drained that birdie putt to move one clear of the field after 36 holes. That is a familiar place for Mamat, who shared the second-round lead two years ago en route to a share of eighth place.
'I had good memories from there. That inspired me,' said Mamat. 'I just like to keep the good thoughts in my head. You never know what will happen, so we will see.'
Marksaeng flew out of the gate on Friday with five birdies in his first six holes, then traded two birdies and two bogeys over his next holes. He bogeyed 16, but the real trouble came at the par-3 17th.
His tee ball landed in the water and Marksaeng struggled from there. He three-putted for a triple-bogey, but rebounded to birdie the last and get into a share of second place.
'I'm a little disappointed,' said Marksaeng. 'My palms were sweating and when I hit the tee shot, I knew it was headed into the water. I was happy though to birdie the last hole after that difficult 17th. I'm playing well enough to challenge.'
Both mixed an eagle, four birdies and two bogeys for his second-round 68.
Ahmad Dan Bateman (69), Unho Park (72), Ross Fisher (68), Peter Hanson (69) and defending champion Nick Dougherty (70) are knotted in fifth place at minus-5.
'There is still plenty of time to get back into it, but I am making it harder for myself than I need to,' said Dougherty. 'There is a good chance if the leader is a guy who hasn't won before, especially if it is an Asian player chasing a European Tour exemption, it might put enough pressure on him to stop him pushing on and getting to a score I cannot reach.'
Stuart Little (69), Barry Lane (69), Boonchu Ruangkit (73), Gary Evans (72), Charl Schwartzel (69), Simon Dyson (67) and Michael Hoey (65) are tied for 10th place at 4-under-par 140.
Marcus Fraser, who shared the opening-round lead with Mamat, struggled to a 4-over 76 on Friday. He dropped into a share of 17th at 3-under-par 141.
The 36-hole cut fell at one-over-par 145 and 80 players advanced to the weekend. Niclas Fasth, the highest-ranked player in the field at No. 57, missed the cut as he finished at 3-over-par 147.