Stuttgart: Top seed Simona Halep, a semi-finalist last year at the Stuttgart Open, battled to a 4-6, 6-2, 6-3 second round win over Slovakia's Magdalena Rybarikova on Wednesday.
The 26-year-old Romanian relied on her ferocious groundstrokes to secure a decisive break in the sixth game of the final set and went on to book a spot in the quarter-finals.
Halep also overcame what appeared to be a hip problem in the first set but did not require a medical timeout.
Russian Anastasia Pavlyuchenkova battled past US Open runner-up Madison Keys 7-6(7), 5-7, 6-4. The 26-year-old will take on second-seed Garbine Muguruza in the second round.
American CoCo Vandeweghe produced a merciless display to beat compatriot Sloane Stephens 6-1, 6-0.
Vandeweghe, who had teamed up with Stephens last weekend to help the US defeat France and reach the Fed Cup final, was broken to love in the first game by the US Open champion.
Yet Stephens dropped her own serve in the next game as Vandeweghe reeled off 12 consecutive games to wrap up victory in 56 minutes in the opening round clash at the Porsche Tennis Grand Prix.
Vandeweghe, ranked 16 in the world, recorded her first victory over Stephens on clay, having lost their only previous encounter on the surface in Strasbourg in 2015.
"I really don't like clay, it's my least favourite surface," Vandeweghe, who will meet Laura Siegemund in the next round, said after the match.
"I'll never like it. This is my second time in Germany " I'm not here for the clay, it's more the shopping. I didn't come out here to play and lose."
Former world number one Angelique Kerber avenged last weekend's Fed Cup defeat to collect a 6-3, 6-2 win over Czech Petra Kvitova in a first round match.
The German will meet Anett Kontaveit in the last-16 after the Estonian saved two match points to knock out last year's runner-up Kristina Mladenovic 5-7, 7-6(3), 7-6(5).
In other first-round results, Russian qualifier Veronika Kudermetova surprised Spain's Carla Suarez Navarro 7-6 (5), 6-2 and Kazakhstan's Zarina Diyas downed Germany's Carina Witthoeft 6-3, 6-2.
With inputs from agencies