Mixed fortunes for Williams sisters in Madrid
Serena Williams marked her first European appearance of the season with victory on the Madrid Open's striking blue clay courts, but her sister Venus crashed out in the second round.
Defending champion Petra Kvitova also started well as the third seed defeated New Zealand's Marina Erakovic 6-2 6-3.
Towering Czech Kvitova beat Victoria Azarenka in the final last year before going on to win the Wimbledon title.
The only seeded player to fall was No 15 seed Jelena Jankovic who went out to Spanish wildcard Carla Suarez 4-6 7-6 6-4 in the first round.
Serena Williams, who reached the third round the last time she played in Madrid in 2010, powered past last week's Budapest Grand Prix runner-up Elena Vesnina of Russia 6-3 6-1.
The ninth seed delivered some powerful serves, hitting 14 aces to set up a meeting with another Russian, Anastasia Pavlyuchenkova, in the next round.
''I was feeling good,'' Williams told reporters.
''It's my first tournament in Europe this year and I just hope I can keep up the good form,'' added the American who has yet to lose in eight clay outings this season.
''I haven't noticed a difference between the blue and the red clay. I think it's the same, it's just you don't get as dirty,'' added Williams.
Venus entered Madrid on a wildcard as she started her third tournament after seven months out suffering with Sjogren's Syndrome, an auto-immune disease which causes fatigue and joint soreness.
The former world No 1 struggled past Romania's Simona Halep in three sets on Saturday, but crumpled against stronger opposition on Monday falling 6-4 6-1 to 12th seed Angelique Kerber of Germany.
Kerber, 24, is in the midst of her breakout season with both her career title coming in 2012
China's Li Na, the eighth seed, joined Kerber in the last 16 with a straightforward victory over Spanish wildcard SilviaSoler-Espinosa 6-3 6-1.
Czech qualifier Andrea Hlavackova and compatriot Lucie Hradecka, Italian Sara Errani, Russia's Ekaterina Makarova, and Estonia's Kaia Kanepi also made it through to the second round.