Jonas Hofmann and Guido Burgstaller score the goals as the all-German tie sits delicately poised ahead of the return leg at Borussia-Park.
There was nothing to split Bundesliga rivals Schalke and Borussia Monchengladbach on Thursday, as the first leg of their Europa League last-16 tie finished in a 1-1 draw at the Veltins-Arena.
The second meeting of the clubs in the space of five days - Gladbach won 4-2 in Saturday's Bundesliga clash - produced a tale of contrasting halves, with both goals coming in a pulsating first 45 played underneath the stadium's closed roof.
Jonas Hofmann's first Gladbach goal put the visitors in front a quarter-of-an-hour into the contest in Gelsenkirchen, only for Schalke to respond with a well-placed finish from Guido Burgstaller 10 minutes later.