ORLANDO, 6 Dec 2014 (AFP) - The 21-year-old American stood seven strokes ahead of second place Henrik Stenson and Keegan Bradley, a Challenge record 54-hole edge, with Woods 20 adrift.
"That's the lowest score I've ever shot after three rounds," Spieth said. "I couldn't ask for better than that."
Tiger Woods battled through nausea, fever and vomiting to fire a 3-under par 69 Saturday at the Hero World Challenge, his first tournament after a four-month injury layoff.
The former world number one birdied the last three holes to stand on level par 216 for 54 holes at Isleworth but remained last in the 18-player field despite shooting lower scores this week the sicker he has felt.
"It wasn't easy. I fought hard," Woods said in a hoarse whisper. "Spent all I had."
Woods, who had not played competitively since the PGA Championship in August because of a back injury, vomited from the first hole onward, making six birdies and three bogeys on the day.
"I've been throwing up for hours," Woods said, noting he has unable to keep down food or sleep well but was not in pain and never considered quitting.
"I wasn't doing too good at the beginning but I thought I could hang in there," Woods said. "The fever just broke. As the round went on I was starting to feel better."
Woods, who opened with a 77 Thursday, fired a 70 Friday with a fever but was worse off Saturday.
"I didn't have nausea and vomiting before," Woods said, adding that it left him weaker.
"I just didn't have the explosiveness," he said. "I was pretty slow."
The 14-time major winner, chasing the career record of 18 majors won by Jack Nicklaus, has only been hitting balls for a month and has again revamped his swing by returning to a style from younger days.
Woods turns 39 later this month -- and no player has won four majors beyond that birthday.
Scores after Saturday's third round of the Hero World Challenge at Isleworth (USA unless noted, par 72):
196 - Jordan Spieth 66-67-63
203 - Keegan Bradley 72-66-65, Henrik Stenson 67-68-68
205 - Patrick Reed 73-63-69
206 - Justin Rose (ENG) 72-64-70
208 - Jason Day (AUS) 71-67-70
209 - Graeme McDowell (NIR) 68-73-68, Matt Kuchar 69-70-70, Rickie Fowler 67-70-72, Bubba Watson 69-68-72
210 - Zach Johnson 67-71-72
212 - Billy Horschel 73-72-67, Hideki Matsuyama (JPN) 68-73-71, Chris Kirk 70-68-74, Jimmy Walker 68-69-75
213 - Hunter Mahan 71-71-71
214 - Steve Stricker 67-73-74
216 - Tiger Woods 77-70-69