Serena Williams beat Elena Vesnina 6-3, 6-1 to reach the Madrid Open's second round, while sister Venus lost 6-4, 6-1 to Angelique Kerber on Monday.
The ninth-seeded Williams had 13 aces and 40 winners against Vesnina, who was struggling with her game on the slippery blue clay court. The Russian lost her balance in the sixth game and set up a break point for Williams, who pulled ahead to lead 4-2.
Williams' ninth ace of the match provided a set point for the ninth-ranked American to take the first set and wrap up the match against the 68th-ranked Russian in just over an hour.
''I'm feeling good but I wasn't thrilled,'' said Williams, who will face Anastasia Pavlyuchenkova of Russia. ''This is my first tournament in Europe so hopefully I will just keep my good form and just keep going,'' she added.
Venus Williams' third tournament since being sidelined by illness is over after being broken six times by the 12th-seeded German. The seven-time major champion reached the quarterfinals at Miami and Charleston, S.C., earlier this season.
Fifth-seeded Samantha Stosur rallied to beat Christina McHale 2-6, 6-4, 6-0 to reach the third round, as did French Open champion Li Na after winning 6-3, 6-1 against Silvia Soler-Espinosa.
Petra Cetkovska also advanced, beating Lourdes Dominguez Lino 6-4, 7-5.
Defending champion Petra Kvitova beat Marina Erakovic 6-2, 6-3 in her first-round match.
Kvitova needed over an hour to beat Erakovic, with the Czech player saving all three break points against her while converting four.
Also on Monday, Andrea Hlavackova, Ekaterina Makarova, Lucie Hradecka, Lucie Safarova and Sara Errani won their first-round matches.
Local favorite Carla Suarez Navarro rallied to beat 15th-seeded Jelena Jankovic 4-6, 7-6 (5), 6-4 to reach the second round.
In the men's tournament, Spain's Nicolas Almagro rallied to beat Santiago Giraldo of Colombia 6-7 (5), 6-3, 6-3.
Almagro saved four set points before hitting long to lose the first set in a tiebreaker. The thirteen-ranked Spanish player broke his opponent in the opening game of the second after adjusting his game to the new court surface to win the match in three sets.
''The court isn't very good, it's very slippery,'' Almagro said of the unorthodox blue clay court that has been criticized by players after it was introduced for the first time in this year's tournament.
Other winners Monday included Serbia's Victor Troicki, Mikhail Youzhny of Russia and American Ryan Harrison who beat Sergiy Stakhovsky of Ukraine, 7-6 (3), 7-6 (5).
Guillermo Garcia-Lopez of Spain ousted 2006 Australian Open finalist Marcos Baghdatis of Cyprus 6-4, 3-6, 6-4.
Top-ranked Novak Djokovic begins his title defense Tuesday against Daniel Gimeno-Traver, who beat Victor Hanescu 7-6 (3), 4-6, 6-3.
