After competing in his first European Tour event for more than five months following chronic back trouble, he said: "I'll settle for that after such a long lay-off. It was a great start and I can't wait for my next event in Madrid in a couple of weeks."
The Macclesfield professional was surprised how well he played over the first two rounds when he shot 69 and 65 to lie in fourth place on 10-under-par.
"I improved by another two shots early on the third day at Carnoustie and was flying but then I got a bit ahead of myself and started to drop a few," he said.
"It was a tough course and I finished with a one-over 73."
He closed with 74 at St Andrews, seven-under-par all told and 12 shots behind winner Stephen Gallacher who birdied the first extra-hole of a sudden-death play-off to defeat Northern Irelands Graeme McDowell.
Birdie
"I played with Darren Clark for the first time and I beat him by one," added Donaldson.
Gallacher, nephew of former Ryder Cup captain Bernard, put on a sensational display at the Home of Golf.
A final round of 67 set the clubhouse target of 19-under that only McDowell could match with a closing 68. That took the contest into extra holes and, much to the delight of the home crowd, the Scot played a stunning approach to three feet for a winning birdie.
"It's a dream come true," said Gallacher. "To win my first title here at St Andrews is fantastic."
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