Gaizka Garitano's men were in control for most of the game, crafting several big chances and having two goals disallowed, but only take a slender victory to Granada for the second leg.
FULL TIME I Concludes the first match of the semifinals of the #CopaDelRey in La Catedral. Athletic will travel to Granada with an advantage goal after a great effort in San Mamés.— Athletic Club (@Athletic_en) February 12, 2020
💪 Goazen Athletic ‼️
1-0 I #AthleticGranada 🏆 #AthleticClub 🦁 pic.twitter.com/YP2ricfcFr
It came as little surprise when Athletic went in front in the 42nd minute, as Iker Muniain tapped home after fine work from Inaki Williams on the left flank.
Athletic remained the dominant side after the interval, but twice offside calls prevented them doubling their advantage, giving Diego Martinez's Granada hope for March's return encounter.
After an even start, Athletic began to carve through Granada regularly and went agonisingly close to the breakthrough in the 15th minute, but Rui Silva tipped Wiliams' fierce drive wide.
Two chances then fell to Mikel Vesga soon after – Silva palming his wicked long-range strike away from the top-left corner, before then blocking a tamer effort from inside the box a few minutes later.
Athletic eventually got the deserved opener just before half-time, the lively Williams brilliantly bringing down a lofted pass with his chest and then showing great composure to tee up an easy close-range finish for Muniain.
The hosts were twice denied a second goal inside the first 15 minutes of the second half, with Yeray Alvarez's strike chalked off for offside before Ander Capa's 20-yard volley was disallowed after a VAR review adjudged the offside Williams to have obstructed Silva's sight.
Granada began to offer some attacking threat towards the end, with Unai Simon turning Carlos Neva's drive around the post and then catching Maxime Gonalons' header from the resulting corner, as Athletic held on.
What does it mean?
Athletic have the aggregate lead, but Granada are not to be underestimated and represent formidable opponents in their own stadium.
In 12 home matches across all competitions this season, Granada have won eight. Three of the four teams to take something away from the Nuevo Los Carmenes are Sevilla, Atletico Madrid and Real Sociedad, all of whom are in the top six.
When there's a Williams, there's a way
More or less everything good about Athletic's play went through Muniain or Williams, or both, but the latter was the standout performer. He gave the Granada defence so many issues with his pace and movement, and his role in the winning goal was just sublime.
Granada struggled immensely to offer any form of attacking threat, with Carlos Fernandez and Roberto Soldado provided with poor service. Darwin Machis should have been one of the players supporting them, but he touched the ball just 13 times before his 71st-minute withdrawal, failing to create a single chance.
Athletic welcome Osasuna on Sunday as they return to LaLiga action, having not won any of their past eight league matches. Granada play host to Real Valladolid the day before. The two sides resume hostilities in the second leg at the Nuevo Los Carmenes on March 5.