Berlin Open top seeds Ons Jabeur and Maria Sakkari progressed with relative ease from their first-round matches, but Aryna Sabalenka and Garbine Muguruza were not so fortunate.
World number four Jabeur, who will team up with Serena Williams at the Eastbourne International, suffered a shock first-round exit at the French Open, but the Tunisian showed no signs of that defeat having affected her in her first grass-court appearance of the season.
Top seed Jabeur required just 73 minutes to overcome Karolina Muchova 6-3 6-3 on Tuesday, with American qualifier Alycia Parks next up after she defeated Qinwen Zheng in straight sets.
Second seed Sakkari, meanwhile, had a similarly comfortable start to her campaign, beating Leolia Jeanjean 6-3 6-2.
Meanwhile, reigning champion Liudmila Samsonova took her place in round two with a 7-5 6-1 victory over Tamara Zidansek.
Samsonova beat Belinda Bencic to claim the title in 2021, and the pair will meet again the quarter-finals should they win respective second-round ties against Veronika Kudermetova and Anna Kalinskaya.
But Kudermetova goes into her clash with Samsonova on a high, having eliminated third seed Sabalenka in a 2-6 7-5 6-4 comeback victory.
WTA Finals champion Muguruza will not be in round two after the Spaniard fell foul of Germany's Andrea Petkovic, who prevailed 7-6 (10-8) 6-4.
"I really do believe that in these really close matches, the simple and little shift of energy can make the difference," Petkovic said.
"If you have the crowd on your side, and they are willing you to the win somehow, I think that can really make a difference."
At the Birmingham Classic, top seed Jelena Ostapenko booked a second-round meeting with Dayana Yastremska.
While Yastremska beat Magda Linette in straight sets, Ostapenko was made to work, with the Latvian requiring two hours to overcome Canada's Rebecca Marino 6-2 5-7 6-3.
Fourth seed Elise Mertens was ditched out by Caty McNally, while former world number two and two-time major champion Petra Kvitova lost to Beatriz Haddad Maia.
Camila Giorgi, the third seed, leads Tereza Martincova after claiming a close first set 9-7 in the tie-break before the match was suspended.