They hold a three-stroke lead over the chasing pack of five which includes Denmark's Thomas Bjorn, winner this year of the Qatar Masters, and another Spaniard Pablo Larrazabal.
Foster, the first round leader, went into the third round one shot behind Garrido.
A birdie on the par-5 second was followed by an eagle at the ninth as the 36-year-old Englishman, looking for only his second win on the European tour, went out in 33.
A bogey at the 11th checked his progress but two further birdies gave him a round of 68 and a 54-hole total of 205.
The 39-year-old Garrido also enjoyed a fruitful front nine on the course which will play host to the 2014 Ryder Cup, shelling three birdies as he also turned in 33.
Like Foster, he dropped a shot at the awkward 11th as well as at the 15th but birdies at 14 and 16 gave him a round of 68 and an excellent chance of landing his first title since 2003.
Foster and Garrido hold a three-stroke advantage over Bjorn, Larrazabal, the Austrian Bernd Wiesberger, Anthony Kang of the US and Ireland's Peter Lawrie.
The best rounds of the day came from Scotsman Stephen Gallacher and South Africa's George Coetzee who both went round in 67. Gallacher is in a group of five who are four shots off the lead while Coetzee - who has recovered superbly from a horrific first round 77 — is one shot further back.
In Edison, New Jersey, what once looked to be a long week at The Barclays suddenly has turned into a sprint.
Matt Kuchar heard the news from his walking scorer when he reached the eighth green Friday that the PGA Tour’s opening playoff event would be reduced to 54 holes on Saturday because of Hurricane Irene. By then, the defending champion had already left his mark on Plainfield Country Club with a 6-under 65.
That gave Kuchar a one-shot lead at 14 under over Dustin Johnson and Vijay Singh, and it could loom large.
Even now, no one is sure what to expect from Irene. The plan is to start the third and final round Saturday morning and hope to finish before the storm arrives. If the rain shows up early and they can’t finish, it will revert to a 36-hole tournament.
Johnson missed birdie putts inside 10 feet on his last three holes and still shot 63. Singh, who has gone 67 starts on the PGA Tour since his last win at the BMW Championship in 2008, two-putted for par from 40 feet on his last hole for a 64.
It’s the first time a FedEx Cup playoff event has been reduced to 54 holes since the series began in 2007, and there was not much choice. Plainfield already had received about 10 inches of rain the past two weeks and 10 more were expected Sunday. There also were safety issues. Crews began dismantling electronic scoreboards Friday afternoon and an army of volunteers from the area had to make their own plans to evacuate, if necessary.
