Sixth seed Venus Williams served 12 aces to beat Kateryna Bondarenko 6-4 6-2 and advance to the quarter-finals of the Porsche Grand Prix.
Williams went 3-1 down but recovered to break serve twice and win the first set before romping home in the second.
Jelena Jankovic beat Bondarenko's older sister Alona 6-2 6-0 to advance.
It was the Serb's first match since learning she will regain the world number one spot from Serena Williams after the tournament in Stuttgart.
Jankovic and Kateryna Bondarenko traded breaks of serve at the start of the match but once Jankovic broke for a 3-2 lead she did not lose another game.
"I am pretty happy with the way I played, it was my first match indoors [this season]," said Jankovic, who lost the US Open final to Serena.
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"I'm playing better and better, I am improving. I don't feel any extra pressure, if anything (I have) extra motivation."
Russian third seed Dinara Safina beat Bulgaria's Tsvetana Pironkova 6-1 7-6 (8-6) for a quarter-final place and a clash with Venus.
Safina trailed 3-5 in the second set after having her serve broken but broke back immediately to love. Safina never trailed in the tie-break and converted her second match point.
Safina can still finish the year as world number one after winning four titles and finishing runner-up at the French Open and the Olympics and making the semi-finals at the US Open.
Two other Russians, Vera Zvonareva and Nadia Petrova, also advanced to the last eight.
Seventh-seeded Zvonareva spoiled Marion Bartoli's 24th birthday by beating the Frenchwoman 6-2 6-0, while Petrova overcame Patty Schnyder of Switzerland 6-4 6-1.
Petrova, the 2006 champion in Stuttgart, will meet Li Na of China in the quarter-finals while Zvonareva will meet Jankovic.
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