Jelena Jankovic beat Samantha Stosur 6-4, 6-4 to advance to the second round of the Porsche Grand Prix on Wednesday.
But another former No. 1 in the world, Caroline Wozniacki, lost in the first round, falling to Carla Suarez Navarro of Spain 7-6 (6), 6-1.
The former top-ranked Dane squandered a 3-0 lead in the first set and never recovered after losing the tiebreaker.
Jankovic now has a 6-2 career lead over Stosur, though she had lost to the Australian the last time they played on clay, in the semifinals of the 2010 French Open.
"I've had some good results this year," the Serb said. Jankovic won in Bogota and was runner-up in Charleston.
Jankovic broke serve for a 5-4 lead but struggled to serve out the first set. She also wasted two match points at 5-3 on Stosur's serve in the second, before using her third match point on an error by Stosur.
The Stuttgart tournament is played on indoor clay.
Also, fifth-seeded Petra Kvitova of the Czech Republic, a former Wimbledon champion, beat wild-card entry Annika Beck of Germany 7-5, 6-7 (0), 6-3 after breaking serve for the second time in the final set for a 5-2 lead. Lucie Safarova of the Czech Republic beat Mona Barthel of Germany 6-4, 6-1.
Barthel said she was tired after helping Germany defeat Serbia in the Fed Cup playoffs for a place in the World Group.
Safarova won nine of 10 games from 4-3 down. She also played Davis Cup over the weekend, losing in Italy.
"Today's match was a good one to get through," Safarova said. "I was playing against an opponent who had been practicing here on the center court in the lead-up to the Fed Cup tie on the weekend.
"It took me a few games to get my rhythm because the conditions here are so different to Fed Cup in Palermo, where the surface was very slow - here it's much faster and it took a while to get used to," Safarova said.
Julia Goerges of Germany, who beat Wozniacki for the 2011 title in Stuttgart, advanced to the second round when Kirsten Flipkens of Belgium retired with stomach problems. Goerges was leading 6-2, 2-1.