By day's end, that lead was gone -- and things had tightened up considerably for Sunday's final round.
Pettersson and Henrik Stenson combined to shoot an 8-under 63 for Sweden, which is seeking its first World Cup title since 1991, nine years before the event was added to the World Golf Championships schedule.
'We were trying to grind,' Pettersson said. 'We knew we had to shoot low, and on the back nine we really got it going.'
Argentina's Andres Romero and Angel Cabrera saw their run of 42 straight holes without a bogey end at Saturday's eighth hole and stalled somewhat from there, yet still managed a 67.
Colin Montgomerie nearly holed out his approach at the 14th, with the ball rolling to a stop just inches from the cup to set up a birdie -- one of four in a row -- for Scotland, which shot 65 and finished 14 under, two behind the Swedes.
After the Scots, it's a seven-way tie for fourth at 11 under -- with Wales (62) and the United States (63) making big third-round moves to get into that cluster, along with Italy (64), Mexico (65), Spain (67), South Africa (67) and Germany (68).
'I think it might be a dogfight tomorrow,' said Stewart Cink of the U.S.
Cink and J.J. Henry closed the front nine with seven straight birdies, then made only one more on the back nine -- with no bogeys on their card.
Good as the Americans' round was, it wasn't even the best of the day.
Bradley Dredge and Stephen Dodd, the defending champions from Wales, recovered from Friday's alternate-shot 75 with a 62, closing with five straight birdies to enhance their bid become the first team to win two straight World Cups since Fred Couples and Davis Love III won four straight from 1992-1995.
'Had a good day today, which doesn't make up for yesterday, but certainly better than we did yesterday,' Dodd said.
Still, they face a five-shot deficit and many teams in the title mix.
'Have to have a brilliant day,' Dredge said.
While many teams made moves -- Switzerland and Australia each shot 64 and France shot 65, although those teams are well off the lead entering Sunday -- one of the pre-tournament favorites never got going. England's team of Luke Donald and David Howell managed only two birdies and one bogey Saturday, finishing at 7 under for the tournament, nine shots behind the Swedes.
'We both played pretty poorly, really,' Howell said. 'We only had one birdie each, which isn't good enough, especially in fourballs. We scrapped it around at level par, which is not what we needed.'
The teams play foursomes, or alternate shot, Sunday to decide who'll split the $1.4 million first prize.
Argentina entered the day three shots ahead of the field and was five shots ahead after four birdies in Saturday's first seven holes.
But after the bogey on the par-3 eighth, Romero and Cabrera had only one more birdie the rest of the way and Sweden took advantage.
Pettersson -- who won The Memorial earlier this year and earned more than $2.6 million on the PGA TOUR -- made Sweden's fourth straight birdie at the 12th, pulling his team into a share of the lead. Pettersson later added birdies at Nos. 15 and 16, the second one giving the Swedes outright control of the lead, which they held the rest of the day.
'We always felt we'd have a chance,' Stenson said. 'And obviously, we've put ourselves in a good position.'