After Barcelona withstood some early pressure from United, Messi ended any hopes of a comeback for the Premier League side as the hosts advanced 4-0 on aggregate.
Messi opened the scoring with a brilliant solo effort in the 16th minute after he won possession of the ball, ghosted past Fred and Phil Jones, and curled a shot into the far corner.
The second came just four minutes later, although Messi was helped by a massive error by David de Gea that time. The Argentina forward only managed a tame shot on goal but the Spain goalkeeper let it slip underneath him and into the net.
Philippe Coutinho added a third goal with a superb curling strike from long range in the 61st to cap arguably the best performance by the former Liverpool player since joining Barcelona just over a year ago.
“It was spectacular,” Messi said after the win. “This is what we are. In the first five minutes, we went in a little cold and nervous – but from then on the goals came and we put in a spectacular performance.”
Manager Ernesto Valverde added that his side were now “confident” of reaching the final in Madrid. Barcelona will play either Liverpool or Porto for a place in the final.
“We are very excited,” he said. “It's been a while since we were in the semi-finals – now we'd love to get to the final. We are confident, but we know that five bad minutes can leave you eliminated.
“[Messi] played great football and his goals changed the way the game was going. But I'm not going to apologise to our rivals because we've got Messi and they don't!”
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