Belgium’s Elise Mertens set up a last 16 clash against world number one Iga Swiatek at the Porsche Grand Prix in Stuttgart after defeating Tatjana Maria on Tuesday. Mertens notched a 6-1, 4-6, 6-0 win over the German and will face two-time defending champion Swiatek for the first time. Australian Open finalist Zheng Qinwen also got off to a strong start with a 6-2, 6-3 win against Sorana Cirstea and advanced to the last 16 for a second straight year. Zheng had a long trip from China to Germany, but showed no signs of jet lag. “It was not easy, but I sleep on the plane, so the time passed fast,…