Naomi Osaka continued her recent good form as she beat Jennifer Brady to advance to the second round of the Volvo Car Open in Charleston.
The Japanese won the BNP Paribas Open at Indian Wells last month and, on the day she reached a career-high of 21st in the world rankings, the 10th seed defeated American Brady 6-4 6-4.
She will next face German Laura Siegemund, a 7-5 6-2 winner over Natalia Vikhlyantseva.
Osaka was the highest seed in action on Monday and there were no upsets for the other seeded players in the draw.
Daria Gavrilova (11) fought back to beat Ons Jabeur 1-6 6-4 6-3, 12th seed Kiki Bertens was a 6-4 6-1 winner over Veronica Cepede Royg, Alize Cornet (14) triumphed 6-1 6-2 against Kateryna Bondarenko and Mihaela Buzarnescu - seeded 15th - won the first set against Vera Lapko 6-4 before her opponent retired in the second.
Hungary's Fanny Stollar beat American Francesca Di Lorenzo 7-5 6-1 and she will next face Great Britain's Johanna Konta.
Meanwhile in Monterrey, third seed Lesia Tsurenko extended her winning streak in a hard-fought victory over Nao Hibino at the Abierto GNP Seguros.
Tsurenko, who has now won 11 in a row, went to three sets to overcome Hibino 6-7 (2/7) 6-4 6-4 in two and a half hours, while second seed Magdalena Rybarikova brushed aside Risa Ozaki 6-4 6-3.
Sixth seed Ana Bogdan had to fight back to beat local favourite Victoria Rodriguez 4-6 6-4 7-6 (7/5), while seventh seed Sachia Vickery was a 6-1 6-4 winner over Irina Falconi.
There were also wins for Ajla Tomljanovic and Alison Riske.
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