Guedes' thundering effort at the end of the first half decided the in shape at Mestalla and the tie after the first leg had finished 1-1 at San Mames ultimate month.
Valencia will meet both Real Betis or Rayo Vallecano inside the final in April, with Betis seeking to capitalise on a 2-1 gain once they play the second one leg at domestic on Thursday.
With Real Madrid, Barcelona, Atletico Madrid and Sevilla all suffering early exits, Betis are arguably favourites now to win the opposition. They are the very best-located La Liga facet left, sitting 1/3, six spots above Valencia in 9th.
But after defeating Athletic, Valencia will fancy their probabilities of clinching a 9th Spanish Cup.
They are the most latest winners ultimate, once they beat Barcelona to raise the trophy in 2019, when Marcelino, now Athletic educate, turned into in charge.
Athletic had knocked out each Madrid and Barca en course to this year's semis however regarded worn-out and flat against a extra competitive Valencia side, and omit out on a 3rd consecutive final.
Guedes' strike changed into a worth aim to decide it, the ball headed out to the midfielder, who become 30 yards out in the forty third minute.
Guedes controlled, shifted right and unleashed a sizzling shot that flew beyond Julen Agirrezabala and into the corner.
