Foster carded a 68 while his close friend Finch went one better with a 67 as both finished on eight-under 136.
One stroke off the pace was Scotland's Paul Lawrie (69), who is certain of his place in next month's European Ryder Cup team, and Australian Brett Rumford (70).
Belgian Nicolas Colsaerts, bidding to beat absent German Martin Kaymer to the one remaining automatic slot in the Ryder Cup side, was three strokes behind Finch and Foster after shooting a 70.
If Colsaerts fails to finish in the top two here on Sunday, Kaymer will be in the team that plays United States in Illinois from September 28-30.
The Belgian, who won the World Match Play title in Spain in May, said he was feeling tired after travelling back from North Carolina on Monday following the weather-interrupted Wyndham Championship.
There was a similarity in the tales of Foster and Finch.
Dismayed after missing the cut in last month's Scottish Open, Foster decided to take a four-week break from golf because he "needed to regroup".
Finch arrived at Gleneagles having missed the cut 13 times this year including his five previous tournaments.
The 35-year-old said he was slightly bemused at the way his form had returned.
"I didn't have the best of starts by making a bogey at the first hole after driving into a bunker," he told reporters.
"But after recent tournaments where I have hit good shots and failed to make birdies and failed to save par, everything seemed to turn around."
Finch, who is 68th on the money-list, holed several long putts including one from 50 feet on the 12th.
Foster, 37, is hoping for a 'Super Saturday' of his own in the third round, three weeks after being in the London Olympic stadium for Britain's greatest night of athletics as Jessica Ennis, Mo Farah and Greg Rutherford all won gold.
"I got some tickets for Super Saturday off a French website and it was probably the best thing I've ever seen," said Foster who occupies 73rd spot on the money-list.
"I took a four-week holiday and practised really hard for one week. I just spent some time at home and sorted the house out too."
Foster has one tour win to his name - the 2003 Dunhill Championship in South Africa.
Finch has won twice before, both times in 2008 at the New Zealand Open and Irish Open.
- Martin Kaymer