The 2012 champion from Scotland had a 12-under total of 128 after starting in morning rain on the 6,848-yard Seve Ballesteros course in the Swiss Alps.
Donaldson had seven birdies and a bogey in his afternoon 64, putting the 38-year-old Welshman in good position to follow up his win at the Czech Masters two weeks ago.
Another American, David Lipsky, shot a 64 and sat a further shot back.
Victor Dubuisson of France, the other Ryder Cup rookie in the lineup, carded a 67 to head a group of players at 8 under.
Edoardo Molinari of Italy, who shared the overnight lead with Ramsay, managed only a 70 and was with Dubuisson and three others, including Scot Marc Warren, whose 63 was the lowest score of the day despite two bogeys early in his round.
The third member of Europe's Ryder Cup team playing here, Thomas Bjorn of Denmark, dropped 10 shots off the lead.
Bjorn, the European Masters champion in 2011 and last year, carded a 72 after his 66 on Thursday.
The cut was made at 1 under, leaving 2010 winner Miguel Angel Jimenez out of the weekend action after a second straight 70.
Jimenez can still stick around to watch the Ryder Cup trio, as he is one of Europe's five vice-captains for the Sept. 26-28 match against the United States at Gleneagles, Scotland.