Tom Lewis turned in 31 to open up a one-shot lead heading into the back nine of the final round of the Portugal Masters.
The Englishman won this event in 2011 in just his third start as a professional and arrived at Dom Pedro Victoria Golf Course in fine form, having finished first and third the past two weeks on the Challenge Tour.
He looked unlikely to contend after an opening 72 but he has birdied 22 of his last 45 holes to get to 21 under and lead the way from Lucas Herbert.
The Australian entered the day with a two-shot lead and turned in 34, making a string of clutch six-to-ten footers to save par.
England's Eddie Pepperell was at 17 under, a shot clear of Ireland's Shane Lowry and Swede Marcus Kinhult.
Pepperell hit the pin as he spun his approach into the second to set up a birdie and he was joined at 16 under by Kinhult who holed a four-footer at the first.
Lewis holed a long putt at the second for a birdie of his own to get within one and when playing partner Herbert bogeyed the same hole, the lead was shared.
The 27 year old made a six-footer on the third to extend that gap to three but Herbert's approach was inside him and he also made a birdie to stay in a share.
Pepperell played a nice chip to birdie the par five fifth for the fourth day in a row and get within two but that was soon three back as both of the leaders made the most of the same hole.
Herbert got a slice of luck when his third hit the pin when it looked certain to race over the green, while Lewis got on in two and two-putted.
Kinhult hit an excellent tee-shot into the par three sixth for a birdie and he was alone in third when Pepperell dropped a shot ahead at the seventh.
The two swapped places again as Pepperell hit a stunning tee-shot to four feet on the eighth and Kinhult bogeyed the seventh to sit alongside Lowry.
The 2012 champion birdied the second, third and ninth to turn in 32 and when he hit an excellent approach into the tenth, he was 16 under.
That was soon five back, however, after Lewis holed an 18-footer on the eighth to get to 21 under and the solo lead.
England's Oliver Fisher - who carded the first 59 in European Tour history on Friday - was at 15 under alongside Finn Mikko Korhonen and South Korea's Soomin Lee.
New Zealander Ryan Fox, Spaniard Sergio Garcia, Finn Kim Koivu and Italian Renato Paratore were 14 under.
