'Three shots is certainly not enough around this golf course,' Forsyth said. 'Hopefully I can keep playing good solid golf and if its not good enough, so be it.'
Sergio Garcia, who held at least a share of the lead throughout the first two rounds, carded an even-par 71 to join Ian Poulter at 6-under-par 207. Christian Cevaer was two shots further back at 4-under-par 209.
Forsyth trailed Garcia by one to start the day at Valderrama and dropped a shot off the bat with a bogey at the opening hole. Garcia struggled early as well with a bogey at the second, but the Spaniard hit his third shot to 6 feet for a birdie at the par-5 fourth to get back to 4 under.
Forsyth persisted and drained a long birdie putt at the seventh to get within one of the Garcia. At the very next hole, Forsyth played his approach inside 14 feet and converted the birdie putt to grab a share of the lead.
Garcia, fueled by the Spanish galleries, knocked his second within 5 feet for a birdie at the ninth to regain the advantage. Forsyth hung around, however, and pitched his third shot to 5 feet for a birdie at the par-5 11th.
Forsyth found himself alone atop the leaderboard after Garcia bogeyed the 12th. At the very next hole, Forsyth landed his second shot inside 25 feet as an unexpected thunderstorm was approaching Valderrama. Forsyth ran home the putt to reach minus-8 moments before the horn sounded signaling a suspension of play.
Garcia had 6 feet left for par at the 13th, and after waiting for over an hour, the 24-year-old returned to the course and missed the putt to fall three shots back.
Forsyth let Garcia climb back, however. After the Scot bogeyed the 14th, Garcia hit his tee shot to 30 feet at the par-3 15th and drained the birdie putt to get within one of the lead.
Forsyth responded with a birdie at the 16th and played a wedge inside 8 feet at the difficult par-5 17th. Forsyth sank the birdie putt to give himself some breathing room with one round to play.
'It would have been a pain to come back tomorrow morning so I was glad to finish, especially with a three-shot lead,' said Forsyth, who last won on the European Tour at the 2002 Malaysian Open. 'I've certainly not been in this positioning a tournament of this stature and I am sure I will be nervous tomorrow.'
Garcia seemed to hit a great third shot to the 17th, but the ball spun away from the flag stopping a good distance away from the cup. He two-putted for par and managed a par at the last as daylight faded.
Peter Hanson posted his third straight round of 70 stand alone in fifth place at 3-under-par 210. Trevor Immelman followed at 2-under-par 211 while Stephen Dodd finished alone in seventh place at 1-under-par 212.
Darren Clarke recovered from a disheartening second round that saw him take an 11 at the 17th with a 68 on Saturday to join Padraig Harrington and Scott Drummond in a tie for eighth at even-par 213.