Forsyth bogeyed the last to shoot a 2-under 69. Ian Poulter fired a 4-under 67 for third place at 4-under-par 138.
Valderrama Golf Club played more difficultly on Friday with intermittent rain and swirling winds. The toughness of the course was no more obvious than Darren Clarke's play on the famous, par-5 17th.
He owned the lead at 3 under par, but laid up short of the green with his third into a fairway bunker. Clarke knocked his fourth into the pond guarding the green and when he walked off the putting surface he left with an 11. Clarke still managed a 1-over 72, but is tied for 25th place at plus-3.
This event is reserved for the top-60 on the European Tour Order of Merit, but several players, including top money leaders Ernie Els and Retief Goosen, along with world No. 1 Vijay Singh, elected to skip the event.
Garcia came to Valderrama and shared the first-round lead with fellow countryman Jose Manuel Lara. Garcia did not look good out of the gate with a bogey at the first and a double bogey at No. 3.
At the third, Garcia thought he lost his first ball so he hit a provisional. His first ball was found, but no one notified Garcia and he missed his 12-foot bogey putt, which was the result of the second ball.
Garcia did not realize that rules officials were looking at the incident to see if there might be a possible problem. If Garcia knew his first ball was playable and he played his second ball, he could have been disqualified.
'It's not nice when you finish and you see an official waiting for you,' admitted Garcia. 'I didn't even know it was for me because I never felt like I did anything wrong.'
Rules officials agreed that no infraction was made.
'Nobody actually came to tell him that a ball had been found and it could have been his original ball,' said John Paramor, the European Tour Chief Referee. 'So he went ahead and played his provisional ball and in these circumstances the score with the provisional ball counts.'
Garcia calmed down after the hiccup at the third with six consecutive pars to close his front nine. He ran home a swinging, 35-foot birdie putt at No. 10, then made it two in a row with a birdie at the 11th.
The 24-year-old, who is ranked 10th in the world, birdied the 14th to reach 4 under par for the championship.
Garcia hit the shot of the tournament at the diabolical 17th. He found the fairway off the tee, then landed on the green with his second shot, some 20 feet right of the cup. Garcia's eagle bid came up short, but he tapped in for birdie to get within one of Forsyth's lead. Forsyth bogeyed 18, so the door was open for Garcia.
He took advantage as his drive settled in the left side of the short grass. Garcia hit his approach 10 feet right of the hole and drained the birdie putt to get the lead midway through the season-ending event.
'It's a shame that it started a bit shaky,' said Garcia, who won his last start on the European Tour at the Mallorca Classic. 'I finally got some putts going on the back nine with some nice birdies.'
Forsyth bogeyed the third, but tallied back-to-back birdies from the eighth, including a 4-footer at the ninth to take the lead at 4 under par. He parred the 10th, but once again reeled off two consecutive birdies from the 11th.
At the 14th, Forsyth landed in the first cut on the left side, then dumped his approach in a greenside bunker. He had a downhill lie and blasted his third 30 feet past the pin. Forsyth's par try rolled 5 feet past the hole, but he made the comeback putt to save bogey.
He rebounded with a birdie at the 15th, but found trouble off the tee at the 18th. Forsyth's drive landed in the trees and he was forced to pitch into the fairway. His third came up 10 feet short of the cup, but his par putt missed left. That cost Forsyth the second-round lead, but insured his spot in the final group on Sunday.
'To be 5 under after two rounds, I must be playing well,' said Forsyth. 'I'm driving it well, finding a lot of greens and any time I'm missing the greens, I'm staying out of trouble and putting well and just have to keep going as well as I can.'
Angel Cabrera fired the lowest round on Friday with a 5-under 66. He is tied for fourth place with Christian Cevaer (70) at 3-under-par 139, one stroke better than Peter Hanson, who shot his second 1-under 70 in as many days.
Brian Davis (73), Jonathan Lomas (72) and Peter O'Malley (72) share seventh at 1-under-par 141. European Ryder Cuppers Padraig Harrington (70), Paul Casey (70) and David Howell (69) joined Terry Price (71) in a tie for 10th place at even-par 142.