Ashleigh Barty, Petra Kvitova and Karolina Pliskova advance in Stuttgart

The defending Porsche Tennis Grand Prix champion Petra Kvitova is through to the quarter-final in Stuttgart, beating Maria Sakkari 6-3, 3-6, 6-3 after two and a half hours.

Petra fended off 13 out of 16 break chances and scored four breaks that carried her home after a titanic battle. The Czech earned a break in the opening set's fourth game and saved three break chances in the rest of the set for 6-3. Sakkari made a quick response in set number two before Petra pulled the break back at 2-4.

Maria created eight break chances in game eight, converting the final one and holding a few minutes later to force a decider. At 3-3 in the final set, Kvitova shifted into a higher gear and won the last three games that sent her over the finish line and into the last eight.

The top seed Ashleigh Barty bested Laura Siegemund 6-0, 7-5 in an hour and 24 minutes, losing serve once and stealing the rival's serve five times to move over the top. Laura fought well in set number two before Ashleigh grabbed a late break for 7-5. Anett Kontaveit toppled the struggling American Sofia Kenin 7-5, 6-4 in an hour and 34 minutes, dominating the return to create 16 chances and converting four to seal the deal in straight sets.