Just 17 seconds separate Roulston from McCauley but the defending champion's team failed to find any chink in McCauley's armour over the 163km of yesterday's seventh stage.
Roulston's lieutenant, Paul Odlin, believes his team leader can still win his third consecutive title.
"Rolly can definitely do it, he has the confidence and the arrogance to do it.
"But it'll be difficult because Gordy is one wee, tough nut to crack, very tough.
"We just have to find a weakness and exploit it." After seven stages, McCauley, of Cycle Surgery, leads on a total time of 17 hours four minutes 50 seconds, with Roulston second at 17sec and Heath Blackgrove, of Colourplus, third at 2min 54sec.
Joseph Cooper, of Subway Avanti, is fourth at 3min 06sec and Delmaine R&R's Dominique Rollin fifth at 3min 07sec.
McCauley, who won the tour in 1996 and 2005, finished eighth yesterday 48sec behind the stage winner, Justin Kerr, of Raboplus, with Roulston two places back at the same time.
Kerr and teammate Peter Latham scored a one-two finish after both riders led a five-strong breakaway from the 30km mark that the peloton let go.
Kerr clocked 4hr 03min 23sec with Latham at 2sec and Subway Avanti's Jesse Sergent third at the same time.
McCauley has clung to the yellow jersey since he nabbed it off Jeremy Yates on the second day of the tour on Tuesday.
Roulston has nibbled away at the lead but his progress was halted yesterday by another lion-hearted display from McCauley which ensured the title fight would go into the final day of the 860km tour.
McCauley dug deep into his reserves yesterday as Roulston probed at what was thought to be his weak point -- in the climbs.
Yesterday's 163.6km stage, from Winton to Te Anau was the second longest of the tour, but contained just a couple of sections in which Roulston could make his play to break McCauley.
Roulston chose to launch his attack on the Blackmount hill section sparking an enthralling battle that lasted about 10km.
The section features three switchback climbs and descents in quick in succession.
Roulston attacked at each climb with riders such as teammate Michael Torckler, king of the mountains leader Jeremy Yates, Australians Benjamin King and Joel Pearson, American Sheldon Denny and Nelson's George Bennett for company.
But McCauley made sure he was with an equally large bunch of riders who included Blackgrove and Rollin, with whom he latched back on to Roulston's group on the descents.
As both bunches came together for the final time with a relatively flat track to the finish, Roulston had run out of aces to play for the day.
Southland Times Trek team manager Ron Cheatley said the plan was to at least nip some time off McCauley's lead but a lack of crosswinds put paid to that.
"It was fairly predictable. It was too calm out there and made it too easy for everybody.
"Rolly tried a few times but Gordy had a few riders around him and chased back up each time.
"We were hoping to gain at least 10 seconds today and we gained none but there's still all to fight for." McCauley said holding the yellow jersey always inspired him.
"It was tough at Blackmount, it was pretty easy until then.
"I managed to hang on to Heath and Dom Rollin and chased back after the climbs. After that all I had to do was stick to Rolly." Odlin said his team's job today was simple -- get the yellow jersey.
"It's two short stages but I think we still can do it." NZPA WGT pm md nb
