Given the stakes involved, Champions League semi-finals don't tend to be fertile ground for managers to learn lessons about their teams.

But for Jurgen Klopp, Tuesday's win over Villarreal provided more than just a ticket to the showpiece fixture of Europe's premier cup competition in Paris later this month.

It also handed the German several important insights into his squad that may prove key to a quadruple bid that was extended by victory in Spain.

Klopp probably did not appreciate the start of that learning process – an unexpectedly poor first-half performance he would prefer to forget.

In possession of 'the most dangerous lead in football', it was key that the visitors did not concede an early goal that would boost the home crowd's belief.

And yet they conspired to do exactly that, twice failing to prevent crosses before Boulaye Dia poked home to ignite El Madrigal.

That early goal set the tone for a first half that saw Liverpool harried (65.5 per cent passing accuracy) and bullied (45.6 per cent duels won) out of the game for large periods.

And so they could have no complaints over going in at half-time level on aggregate fearing that Villarreal would become only the second team to overcome a two-goal deficit in a Champions League semi-final.

Liverpool were, of course, the ones who pulled off that previous comeback, overturning Barcelona's three-goal advantage en route to winning the 2018-19 edition of the competition.

Yet, with their first-half attempts to prevent history repeating itself having failed, it seemed inevitable that changes were coming at the break.

Perhaps the only surprise was that Klopp limited himself to just one alteration, introducing Luis Diaz in place of Diogo Jota but leaving his starting midfield trio alone.

However, it was hard to argue with the results of that minor tweak: a consummate second-half performance that turned a 2-0 deficit into a 3-2 win.

That outcome is good news for Diaz, whose match-high four shots and well-taken goal underlined the brilliance of another sparkling outing.

But it might not be for Jota, who won just two of six duels and completed six of nine passes before getting the half-time hook.

There were also contrasting fortunes in midfield, where Naby Keita provided the most notable recovery from a midfield three that looked lost in the opening 45 minutes.

With Jordan Henderson going through a vigorous warm-up just after half-time, it did not look like the Guinean would last much longer.

But, as was the case with his colleagues in the engine room, he went up a level en route to posting 21 passes in the opposition half, three tackles, and 11 possession regains (more than any other player on the pitch) by full-time.

With the big games coming thick and fast in the weeks ahead, the consequences of these performances are likely to stretch beyond simply securing Liverpool's third Champions League final appearance under Klopp.

As he guides an unlikely quadruple bid towards a dramatic conclusion, the manager now has an even clearer idea of which players he can rely on to deliver on the biggest of stages.