The 7-6 (3), 4-6, 7-6 (1) match was the fourth-longest of the Open Era on the WTA Tour. It was the first to surpass the four-hour threshold since Francesca Schiavone defeated Svetlana Kuznetsova 6-4, 1-6, 16-14 in four hours and 44 minutes at the Australian Open in 2011.
Siegemund rallied from 3-1 down in the first set and 6-5 in the third set. She won nine of the final 10 points to advance.
It was the only singles match played Wednesday because of rain.
Korea Open
Veronika Kudermetova ousted second-seeded Liudmila Samsonova 6-4, 6-2 in an all-Russian encounter in Seoul to reach the quarterfinals.
Kudermetova moved to 5-2 overall against Samsonova, including three wins in the past year. She joins her younger sister, Polina, who already had secured her spot in the quarters.
Another Russian, fourth-seeded Diana Shnaider, defeated Carol Zhao of Canada 6-3, 6-3 in 67 minutes. The fifth seed, Marta Kostyuk of Ukraine, eliminated Great Britain's Heather Watson 6-2, 6-2 in 75 minutes.