Tine Baun, the twice former All-England champion who wanted one last adventure at the tournament which made her famous, caused a major surprise by reaching the final for the fourth time.
It will be the last match of the 33-year-old Dane's career, and she achieved it did that with a dramatic and tortuous 24-22, 19-21, 21-19 win against Sung Ji-Hyun, the fifth seeded Korean, after being 15-20 down in the first game and lasting 76 exhausting minutes with an opponent a dozen years younger.
The seventh seeded Baun also let slip a lead of 19-17 in the second game, an opportunity which might have enabled her to reach the final less tired, and had to endure an agonising court wiping session for two minutes before playing the match point.
That was because Sung had made a full length dive, heroically to return her last shot from the floor, from which sweat had to be removed before the denouement could be completed.
That happened with a perfect overhead drop, a shot which had been an important part of the Baun armoury which included accurate net play and a heavy smash around which she could construct the attacking strategy of her rallies.
Baun took risks to win, and narrowly succeeded. "I wanted to play my way and if I made mistakes, so be it," she said. "This has all been an adventure and I didn't know this could happen.
She will play in the final Ratchanok Intanon, who also caused an upset by winning 21-15, 21-19 against Saina Nehwal, the second seeded Commonwealth champion from India.
Intanon's bold and imaginative play contrasted with a tensed a tired-looking Nehwal, and has already created one record, that of being the first Thai ever to reach the women's singles final. She could become its youngest winner, at 18 years and one month, if she were to succeed again.
Earlier, Lee Chong Wei moved to within one step of winning back the All-England title which helped make him a badminton superstar with an increasingly superb performance which enabled him to win his semi-final in little more than half an hour.
Lee beat the surprise survivor, Tanongsak Saensomboonsuk, 22-20, 21-8 to reach his fifth final and earn a chance of his third title.
Lee had been so disappointed by his patchy quarter-final performance that he said he wanted to forget it, but now in the second game against the Thai qualifier his control, containment, and brilliantly timed counter-attacks were of the highest order.
"I came on to court a little bit nervous because I knew he had been playing very well," said Lee of an opponent ranked outside the top 30 who had come all the way through the qualifying competition.
"There was a lot of pressure. But I feel sharp because I didn't play in Germany and instead concentrated on preparing for the All-England.
"I trained just for the All-England. I have to do things like that more and more because I'm getting a little older and I don't want to get slower."
Lee, who lost last year to Lin Dan, the Olympic champion from China, finds another Chinese player barring his way in the final.
He is Chen Long, winner of the World Super Series finals in Shenzhen, who reached his first All-England final with a perfectly gauged 21-19, 22-20 win over Jan Jorgensen, the unseeded Dane.
Chen was trailing by 13-17 and 16-18 in the first game, and by 9-11 and 18-19 in the second, but stepped up the pace of the rallies tellingly in each game as they reached their crisis moments.