The South Arabia international striker netted twice in the space of four frantic second-half minutes before setting up Thiago Neves for his side's third as 10-man Al Ain, who had goalkeeper Khalid Essa sent off, were left stunned after Tuesday's semi-final first leg at the King Fahd International Stadium.
Neves also missed a penalty after Essa saw red for hacking down Nawaf Al Abid before getting his name on the scoresheet to spark jubilant scenes in front of a packed home crowd.
The result leaves Al Ain with the daunting task of trying to turn the tie around in the second leg later this month, although in striker Asamoah Gyan the Arabian Gulf League can call on the competition's top scorer this season.
Gyan was involved in the first talking point of the game when he appeared to be hauled down in the area by Kwak Tae-hwi, but the officials saw nothing untoward and waved play on.
At the other end, Mihai Doru fired just over the angle of the goal midway through the first half before Al-Shamrani came within inches of breaking the deadlock on the stroke of half-time.
Al-Shamrani exchanged passed with Neves, then surged between two defenders and into the penalty area, only to see his chipped effort come back off the post.
But Al-Shamrani did open the scoring after 61 minutes to kick-start a whirlwind final 30 minutes from the home side.
Al-Shamrani beat his marker on the edge of the area before cutting in at angle and firing over Essa and in off the underside of the crossbar.
It was 2-0 when Al-Shamrani bundled home Abdullah Al Zoari's cross from close range.
Essa was then given his marching orders barely a minute later when he was caught out by Al Abid's quick footwork and crudely brought him down to a concede a penalty.
After a lengthy delay, substitute goalkeeper Dawoud Sulaiman guessed the right way from the resulting spot-kick as he dived low to his right to keep out Neves' spot-kick.
Neves, though, quickly went from zero to hero.
Al-Shamrani pounced on a defensive mistake and his cross eventually fell to Neves, who sent Sulaiman the wrong way with his shot from 10 yards.
