Top-seeded and defending champion Maria Sharapova and second-seeded Australian Open runner-up Li Na were among Thursday's second-round winners at the $795,000 Porsche Tennis Grand Prix.
The former world No. 1 and reigning French Open champ Sharapova held off Czech Lucie Safarova 6-4, 6-7 (3-7), 6-3, while the former Roland Garros titlist Li leveled Croatian qualifier Mirjana Lucic-Baroni 6-1, 6-2 on the red clay at Porsche Arena.
The Porsche global brand ambassador Sharapova beat Victoria Azarenka in last year's marquee finale in Stuttgart.
The Russian superstar will meet fellow former No. 1 Ana Ivanovic of Serbia in Friday's quarterfinals.
Li's quarterfinal opponent on Friday will be fifth-seeded former Wimbledon champion Petra Kvitova, who advanced with a 2-6, 7-6 (7-4), 6-2 victory over German favorite Julia Goerges, the 2011 Stuttgart champion.
In other action featuring seeds, No. 3 Angelique Kerber of Germany handled Russian Anastasia Pavlyuchenkova 6-0, 6-4 and the former French Open titlist Ivanovic took out eighth-seeded Russian Nadia Petrova 6-4, 6-3.
Fourth-seeded Italian Sara Errani, last year's surprise French Open runner-up, was ousted as American Bethanie Mattek-Sands pulled out a 6-0, 4-6, 6-1 win.
Another German victor was Sabine Lisicki, who straight-setted former world No. 1 and 2008 Porsche champ Jelena Jankovic of Serbia 7-6 (7-3), 7-5 in just over two hours.
One other second-round affair saw Kazakhstan's Yaroslava Shvedova drive out Spaniard Carla Suarez Navarro 7-5, 6-4. Shvedova will meet the tough left- hander Kerber on Friday.
The 2013 Stuttgart champ will collect $130,000 and a new Porsche 911 Carrera.