Victoria Azarenka said jetlag left her with "no idea of the time or what's happening" after beating Vera Zvonareva at the Porsche Tennis Grand Prix.
Former world number one Azarenka, who travelled to the Stuttgart event from Australia, won 7-5 6-4 in round one.
The Belarusian said the effects of her flight proved a "huge challenge", adding: "I'm very happy with the way I handled it."
Azarenka, 29, now faces defending champion Karolina Pliskova.
Pliskova - the Czech world number four - is among four of the world's top five players featuring in the event.
Her compatriot and world number three Petra Kvitova, who received a bye to the second round, cruised through to the quarter-finals with a 6-1 6-4 win over Belgian qualifier Greet Minnen.