Top-seeded Garbine Muguruza of Spain and No. 2 Timea Bacsinszky of Switzerland won in straight sets to reach the second round of the Grand Prix SAR clay-court tournament on Tuesday.
Muguruza, a semifinalist last year, saved all three break points she faced in a 6-4, 6-1 win over Daria Kasatkina, breaking the Russian player's serve four times.
Bacsinszky, who has had a breakthrough start to the year, had five aces in a 6-3, 7-5 win against Russia's Evgeniya Rodina to improve to 24-3 overall.
Hungary's Timea Babos upset fifth-seeded Mona Barthel of Germany 6-4, 6-0.
Third-seeded Flavia Pannetta, No. 4 Elina Svitolina, No. 6 Roberta Vinci and No. 8 Monica Puig all advanced to the second round.