The Spain international was in inspired form as his first-half brace on Tuesday evening threatens to end the home side's glorious cup run
Athletic's star striker gave his side the advantage with a header and earned the victory with a well-placed strike ending an efficient individual run. Ander Lambarri gave his side hope for the second-leg with an injury time strike, but the top flight side are in a strong position to reach the final.
Mirandes began the game looking to prove once again that their current campaign has been no fluke. The home side forced Athletic to defend in numbers for the opening minutes, looking to stop them from settling into the flow of play.
Although the Segunda Division B side were controlling the match and outplaying their opponents, they forgot that with just one touch, Llorente can change a game. Dribbling past a defender on the left, Oscar De Marcos reached the touchline and sent a looping cross into the area which the Spain striker easily headed past Nauzet Perez.
Minutes later, he was once again decisive, putting a massive dent into the home side's hopes of reaching the final. Latching onto a pass from the left, Llorente cut into the middle of the 18-yard box, dribbling past two defenders in the process, and managed to beat Nauzet with a weak but well-placed strike to earn a brace on the night.
Carlos Pouso's men nearly conceded a third when an inspirational Llorente found an unmarked De Marcos in the box with a masterful lob, but the pacy midfielder had his resulting strike parried away by Nauzet.
In the 40th minute, Llorente put the ball past Nauzet for the third time of the night only to see play pulled back due to the linesman judging him to be offside, and replays of the goal showed the linesman's decision to be questionable.
The minnows were clearly deflated by the scoreline as they struggled to even keep possession in the second half. Individual runs by Pablo Infante were not enough to threaten the well-organised defence of the away side.
Instead of playing like the courageous and inspired side that eliminated Villarreal, Racing Santander, and Espanyol, Mirandes' began to confirm the difference between their team and Athletic's in the league tables.
But when all hope seemed lost for the minnows, a last gasp goal saw their ambitions of reaching the cup final given a shimmer of light. A long pass was flicked on into the box to find Lambarri who sent his powerful strike sailing past Iraizoz and pull one back in injury time.
But Athletic remain firm favourites to go through, and head back to San Mames with a lead to defend in next week's second leg.