Stenson shot a 68 and was at 138, one stroke better than Lee Westwood (70). Four players were tied at 140.
Casey had a 76 in the first round, apparently battling food poisoning as he slumped several times on his bag. The Englishman entered the final event at Valderrama with a $274,080 lead over No. 2 Padraig Harrington.
Harrington shot a 69 to go with an opening 73, leaving him four behind Stenson. Harrington must finish in the first three to have any chance of overtaking Casey.
Only two other players have a remote chance at the money crown -- No. 3 David Howell and No. 4 Robert Karlsson. They must finish in the top two to have a chance, and both are five shots off the lead. Howell had a 73 to go with a 70, and Karlsson slipped to a 74 after an opening 69.
Stenson, out of contention for the money title, carded five straight birdies on the back nine, pitching in from 25 and 50 yards on two holes.
Casey started his second round with a bogey at No. 1, but recovered at No. 6 with a 15-foot putt for a birdie. He was 1 under for the day before a double bogey on No. 11. Despite bogeys at Nos. 11 and 13, Harrington rebounded with birdies at Nos. 14 and 17.
'I think the cold weather is making it very difficult out there,' Harrington said.
Casey wasn't the only player weakened by suspected food poisoning. Despite an overnight hospital stay, David Lynn shot a 71 to trail Stenson by two strokes. Sergio Garcia was three back after a 70.