Ghana winger Albert Adomah starred for Middlesbrough in their 2-1 away triumph over Brentford in the English Championship Play-offs on Friday night.
The 27-year-old started the game at the Griffin Park before 11691 fans and was booked in the 40th minute.
Jelle Vossen gave the visitors the lead midway through the first half, heading Adam Clayton's cross into the corner.
Andre Gray drew the Bees level when he robbed goalkeeper Dimi Konstantopoulos on the edge of his penalty area and turned the ball into an empty net.
But Amorebieta's well-struck winner, which came after Brentford had dominated the second half, gives Aitor Karanka's side a one-goal cushion heading into the second leg on Friday, 15 May.
Vossen's unmarked header from the middle of the box gave the visitors, who are looking to return to the Premier League for the first time since 2009, the advantage.
The Belgian came close to doubling Boro's lead, latching on to a knock-down in the box and stabbing a shot goalwards but Brentford goalkeeper David Button managed to claw it off the line.
Gray equalised five minutes into the second half after chasing a seemingly lost cause, charging down a clearance from Greek goalkeeper Konstantopoulos and guiding the ball in for his 18th goal of a prolific campaign.
Albert Adomah's left-footed half-volley tested Button from the edge of the area, while Konstantopoulos made amends for his earlier mistake, pulling off a great diving save to turn Jonathan Douglas's looping header behind.
Just as an entertaining and engaging encounter - which contained nine yellow cards - seemed to be heading for a draw, Boro snatched their late winner and a crucial first-leg advantage from a corner.
Venezuelan defender Amorebieta, on loan from Fulham, latched onto a loose ball and steered a left-footed shot past Button in the dying seconds, leading to wild celebrations from the Boro players at full-time.