Real Betis Balompié earned their first win in Group I to keep pace with Olympique Lyonnais at the top of the section, with Álvaro Vadillo's second-half effort proving enough to down Vitória SC.
Struggling domestically, Pepe Mel's men found respite in continental competition as the winger's sublime effort in the 50th minute made the difference. Leading the section going into the match, Rui Vitória's side continued their desperate run on Spanish soil but will be determined to regain top spot in the return in two weeks.
Having lost on all five of previous visits to Spain, Guimarães did not have history on their side but started with a desire to banish that unwanted record. In the first minute, Chris Malonga volleyed over the bar from ten metres, and Fernando Russi later headed a good chance over from a similar distance. After the early storm, Betis worked their way into the ascendency and Jorge Molina saw a powerful low drive excellently kept out by Douglas.
The breakthrough came five minutes after the restart and it was a thing of beauty. Vadillo jinked in from the left flank, exchanged a rapid one-two with Joan Verdú on the edge of the area before opening his body to curl the return perfectly into the far corner with his right instep.
Marco Matias failed to slot in a golden opportunity to equalise, sliding wide from an angle after rounding Betis keeper Stephan Andersen, before the Danish custodian made a remarkable reaction stop from David Addy to secure his side maximum points.