If the Blues can avoid a shock result next Wednesday in the Basque country they will join fellow English sides Arsenal and Manchester City, as well as reigning champions Barcelona in the 18-team league phase.
With Sociedad holding Chelsea to a surprise 2-2 scoreline heading into the final 25 minutes in London, new signing Melvine Malard netted a brace either side of a Lauren James goal to put the hosts in control.
Wolfsburg have one foot in the league phase after goals from Lara Prasnikar and Camilla Kuever gave them a 2-0 home win over 10-woman Inter Milan.
Real Madrid beat Ajax by the same scoreline in the Netherlands courtesy of strikes from Felicia Schroeder and Janou Levels.
Juventus were 3-0 victors in Austria gainst St. Poelten.
Paris Saint-Germain held four-time champions Eintracht Frankfurt to a 1-1 draw in Germany, while HB Koge of Denmark earned themselves a 1-0 advantage in their tie with PSV Eindhoven.
Belgians OH Leuven all but secured their spot with a 4-1 victory at Polish side Czarni Sosnowiec and Sparta Prague's meeting with Servette also produced five goals, the Czechs taking a 3-2 advantage to Switzerland for next week's second leg.
On Tuesday, Brann of Norway beat Austria Vienna 2-1 in Bergen.