So much for that ice-cold German efficiency.

Having squandered a 54-hole lead in the Senior Open Championship for the second year in succession, Bernhard Langer yesterday admitted he had made a serious error of judgment to gift the title to the unsung American Mark Wiebe at Royal Birkdale.

Wiebe took the trophy from under Langer's nose at the fifth hole of their two-man play-off. Having matched each other's scoring over 72 holes of regulation play and four holes of sudden death, Wiebe struck the decisive blow with a sublime approach to the front of the 18th green. Langer, who had come up short, needed a chip and two putts to get down, and Wiebe took the trophy with a calm par.

It was a remarkable triumph for the 55-year-old Oregonian who has rarely set the heather alight over the course of a 33-year professional career in which he had amassed just two wins on the PGA Tour and another three in the seniors' ranks. However, he earned his play-off place with a superb round of 66 on Sunday, a day that had seemed to be a Langer victory parade until the two-time Masters champion stuck his ball in a greenside bunker at the last hole of regulation play.

Langer's fluffed chip from that position – he hit his ball against the face of the trap – led to a double-bogey and a play-off against Wiebe. Following two rain delays, fading light stopped play after two extra holes on Sunday evening, and the two players matched each other stroke-for-stroke again for another two holes when they returned to the course yesterday morning. Then, though, Wiebe struck, taking victory after a weak approach led to a bogey by Langer.

"It was really my tournament to win or lose and I made a major error by taking on the green," said Langer of the 72nd hole mistake that led to the play-off. "I should have just laid up short of the bunkers, chipped it up and made two putts to get out of there with a bogey.

"It almost felt like Jean Van de Velde, if not quite. But that was certainly a bad error and shouldn't have happened as experienced as I am. But we had two rain delays, as you know, and the mind and the brain doesn't always work 100% right."

Wiebe, who had been suffering serious back problems going into the tournament, said: "It's always better to win a play-off with a birdie rather than a bogey, but right now I don't really care. I'm glad it's over. I'm honoured. I'm speechless. Shocked, too. I just planned on Bernie making that putt on the fifth extra hole and I actually was wondering already what hole we were going to go to now.

"Do we go back to 18 or do I start on a new hole? I've never been in a play-off in a major championship. I was a first-timer, so I just tried to play the golf course as well as I could."

Langer's heartbreak followed an equally disappointing finish at Turnberry last year, when he took a one-shot lead into the final round but fell away badly and watched Fred Couples take the title.

However, the 55-year-old German said he was generally satisfied with his performance at a tournament that appeared to be his for the taking until Wiebe snatched the glory at the end.

Langer said: "What I'll take away from this is my tremendous ball-striking. I really drove the ball well this week, hit a lot of good iron shots and thought I played pretty smart – except for the 72nd hole. Otherwise, I played some of the best golf of my whole life."

Wiebe's reward was a winner's cheque for just over £200,000. His victory also earns him a place in the field for next year's Open Championship at Hoylake.