Atlético came in as the underdog against the reigning European champions, who couldn’t find a way to break through the Spanish team’s defense.

MADRID — Atlético Madrid scored early and then defended well to beat Liverpool, 1-0, in the first leg of the Champions League’s round of 16 on Tuesday, spoiling the reigning champion’s return to the place where it had won the European title last year.

Midfielder Saúl Ñíguez netted in the fourth minute,