France’s Romain Langasque, the world No 7 and European No 1, found the cup from off the green to par the 16th and earn a half, and then repeated the feat at the next, this time for a birdie.
It meant the Yorkshireman, ranked No 250 in the European rankings, was taken to a sudden-death play-off where he was beaten at the 19th, ending hopes of an all-English final.
In the other semi-final was Hampshire's Scott Gregory, the man defeated in the English Men's Amateur championship final by Huddersfield's Nick Marsh when the Fixby man won the title in 2014.
Gregory was up against Ireland's John-Ross Galbraith in Seville and prevailed 2&1.
Walker had defeated Spain’s Jorge Maicas Redondo 4&3 in the quarter-finals before facing Langasque, the man who had defeated Walker's Yorkshire team-mate Dan Brown (Bedale) in the first round.
Twenty-one-year-old Walker, son of England men's coach and The Oaks' head professional Graham, placed eighth in England Golf's Order of Merit last year.
On Friday, he beat Ireland’s Conor O’Rourke 3&1 in the first round and was taken to the 18th before defeating France’s Ugo Coussaud one up in his afternoon match.
"He's disappointed, but it's been a good week for him," said Walker snr.