Also on Sunday, Wales edged Bulgaria 1-0 to top the second-tier Group B4, while Ireland lost 1-0 at home to Finland.
Medvedev will next face fellow American Frances Tiafoe, and Thiem will take on Canada's rising star Felix Auger-Aliassime.
"Serena is Serena," Sakkari said after advancing earlier Saturday by beating 19-year-old American Amanda Anisimova 6-3, 6-1. "You have to come up with some great tennis. Otherwise there is no chance against her."
Then in the third set, Williams was rarely troubled by her rival en route to the victory.
Elsewhere, Belarus' fifth seed Aryna Sabalenka lost to compatriot Victoria Azarenka 6-1, 6-3, ninth seed Johanna Konta of Britain was stunned by Romania's Sorana Cirstea 2-6, 7-6 (5), 6-4 and tenth seed Garbine Muguruza from Spain lost to Bulgaria's Tsvetana Pironkova 7-5, 6-3.
"These agents contacted the parents of young players to entice them with seemingly good plans and high compensations, but they seldom fulfill their promises," Liu Baoyu, vice principal of Shandong Luneng football school said.
Meanwhile, Kenya's world marathon record-holder Brigid Kosgei's track debut ended in disappointment after losing the battle to The Dutch world 1,500m and 10,000m champion Sifan Hassan who went to break the World Hour record after she reached 18,930 meters as the hour elapsed, beating the existing mark of 18,517 meters set by Ethiopia's Dire Tune in 2008.
SUZHOU, China, Sept. 6 (Xinhua) -- Beijing Guoan were held to a 2-2 draw by Wuhan Zall in the ninth round of the Chinese Super League (CSL) here on Sunday.
In the other two matches of the day, Hunan Hualai edged Shenzhen Nankeda 3-2, while Olympic champion Chen Long led Xiamen Tefang to a win over Anhui Souyu 5-0. Enditem
Only a couple of hours before the race started on Tuesday, the Tour organizers announced that all the riders' tests returned negative but race director Christian Prudhomme tested positive along with four staff members from four different teams.
MADRID, Sept. 6 (Xinhua) -- World champion Yulimar Rojas of Venezuela extended her season's best to a world-leading 14.71 meters to win the triple jump in a competition in Spain's Castellon.