Trailing 3-0 on aggregate, Divock Origi bought Liverpool hope after just seven minutes when he tapped in the rebound from Jordan Henderson's shot.
There were chances for either side before half-time, but it was Jurgen Klopp's side who then sprang from the blocks in the second.
First it was Trent Alexander-Arnold who broke into space down the right and picked out Wijnaldum, whose low shot squeezed past Marc-Andre Ter Stergen in the Barca goal after 54 minutes.
Then, less than two minutes later with Ernesto Valverde's side still reeling, Wijnaldum steered a great header into the top corner after a pinpoint ball from Xherdan Shaqiri to put the tie level on aggregate at 3-3 with over half-an-hour left to play. Watch the goal below:
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