Liverpool will have to produce one of their best displays of the season at Anfield after Barcelona's Lionel Messi hit them with a second-half sucker-punch in the first leg of their Champions League semi-final.After a frustrating opening half when they fell behind to Luis Suarez's strike, Jurgen Klopp's men gained the upper hand and looked more likely to score the next, with both Mohamed Salah and James Milner testing the agility of goalkeeper Marc-Andre ter Stegen.However, Barcelona showed they can be at their most deadly with their backs to the wall.The home side doubled their lead when Messi followed up after Suarez's strike came off the bar and then the Argentine maestro raised the decibel levels inside a packed Nou Camp with a sublime 25-yard free-kick that found the top-right corner.The Reds came agonisingly close to a crucial away goal when Roberto Firmino's strike was cleared off the line, before Salah struck the post with his follow-up.