Manchester United's Reserves may have trailed to Gorka Guruzeta's stunner for Athletic Club at the break but Ricky Sbragia's side recovered to record a second successive win in the Premier League International Cup with goals by James Wilson and Ethan Hamilton.
Marcos Rojo played the opening 45 minutes to begin his comeback after suffering a serious knee injury against Anderlecht last April.
The hosts started brightly, with Wilson hooking over the bar from a difficult half-chance and Hamilton kept out by Hodei Oleaga at his near post. Yet Bilbao struck first in impressive fashion with a goal of real quality. Demi Mitchell misjudged Inigo Vicente's pass but the punishment was ruthless, Guruzeta leaping to meet the ball on the volley as it bounced up, sending a sublime shot with the outside of his foot past Kieran O'Hara.
Mitchell had a chance to make amends but his free-kick sailed wide and, although Wilson did well to meet Joe Riley's cross with a header that was off target, the visitors began to carry the greater threat. Vicente had three efforts on goal as he showed his class. One drive deflected over the bar, a cheeky chip forced O'Hara to take action and a swerving drive was closer than it initially looked.
Guruzeta and Jon Ander Gonzalez also had attempts in the first half with Rojo heading over the top for United from a Zak Dearnley free-kick when flagged offside in any case. Rojo made way for Tyrell Warren at the interval, with Guruzeta also surprisingly substituted, presumably with a knock.
It took only a couple of minutes after the restart for the Reds to draw level as Dearnley showed persistence down the right wing to find enough space to deliver a cross. Wilson met it with an unerring headed finish past Oleaga to continue his fine form in front of goal.
Athletic Club regrouped with Vicente the dominant performer, feeding Gonzalez on a break, with Luca Ercolani making a good block to thwart the danger. More fine footwork by Vicente ended with a lofted finish over the bar and he then sped clear to dink over O'Hara, only for Warren to clear with a diving header and collide with a post in the process.
Vicente showed a cynical side when tugging Tosin Kehinde back as the midfielder tried to race through, while Scott McTominay blazed over a rare sight of goal for United. However, McTominay was heavily involved in the winner as he showed his determination to prod the ball past Oleaga and into the path of Callum Gribbin. The substitute was charged down but, with the keeper out of position, Kehinde showed excellent composure to tee up Hamilton, who took a touch and buried a left-foot finish.
Tempers flared towards the final whistle and Gribbin picked up a yellow card but Sbragia's men saw the tie out. It leaves the Reds with maximum points from two games ahead of the final group fixture with Celtic next month.
United: O'Hara; Riley, Ercolani, Rojo (Warren 46), Mitchell; Sang, Hamilton; Dearnley (Gribbin 80), McTominay, Kehinde; Wilson (Bohui 74). Subs not used: Richardson, Buffonge, Scott, Whelan.