Porsche Grand Prix, Filderstadt
Kim Clijsters sent out a warning to Justine Henin-Hardenne with a ruthless 6-1 6-1 defeat of Daniela Hantuchova.
Henin-Hardenne could replace her Belgian compatriot as world number one this week unless she reaches the final.
Clijsters' attempt to hold onto her ranking appeared to be dented when she sprained her ankle a fortnight ago.
However the 20-year-old showed no signs of the injury in a clinical performance against an out-of-sorts Hantuchova.
Clijsters said the ankle, which is still iced and taped after practice, is her main concern, not the battle for number one.
"I want to see how it reacts to a match. That's the first thing I will worry about when I wake up tomorrow," she said.
"I don't think the number one is on my mind. I just feel lucky to be able to play this tournament."
Defending champion Clijsters will play Amelie Mauresmo in the third round after the Frenchwoman defeated Patty Schnyder 6-2 7-6 (7-3).
In other second-round matches, Mary Pierce needed six match points before she pulled off the biggest upset of the tournament thus far, ousting world number five Jennifer Capriati 6-4 6-2.
Capriati was unable to cope with her French opponent's big forehand but her biggest problem was exhaustion after a long season that has seen her lose several close matches.
"I wasn't really ready to come here, but it was my duty as a professional," said Capriati.
"It's been a long year for me and I feel I've played a lot of tennis and, to be honest, I'm tired and not in the mood to fight like I always do."
Third seed Lindsay Davenport had no problems against Japan's Ai Sugiyama, winning 6-2 6-4, while Magdalena Maleeva beat seventh seed Chanda Rubin 6-3 6-4.
Russia's Elena Bovina also caused a surprise when she cruised past compatriot Anastasia Myskina 6-2 6-3.
Bovina ended Myskina's nine-match winning streak which took her to two titles in two weeks.
Another Russian Elena Dementieva was given an easy passage into the third round when Silvia Farina Elia retired hurt with the Russian leading 6-0 2-1.
Tashkent Open
Top seeded Petra Mandula was eliminated in the second round, losing to Spain's Arantxa Parra 7-5 6-1.
In another upset, fifth seed Flavia Pennetta lost to fellow Italian Adriana Serra Zanetti 6-4 7-5 but third seed Saori Obata advanced after a 6-1 6-1 win over
Jie Zheng of China.