Mena struck in the 41st and 51st minutes at Estadio Victoria as the visitors secured their fifth win from eight matches.
Sakkari threatened to repeat her come-from-behind victory over Williams in the Western & Southern Open two weeks ago, but ultimately fell just short.
"We played well with counterattacks. We had several chances to score. Krzysztof Piatek had the best chance but Jasper Cillessen saved it," added the 49-year-old.
"In accordance with the Grand Slam rulebook, following his actions of intentionally hitting a ball dangerously or recklessly within the court or hitting a ball with negligent disregard of the consequences, the US Open tournament referee defaulted Novak Djokovic from the 2020 US Open," the statement said.
"It's going to be one of the young guys, I think, if you count Dominic Thiem as a young guy, as well. He obviously has a chance to win. Now I think it is the time where it gets really interesting. We'll see where we go from here." Enditem
Georgia's Iuri Tabatadze(L) vies with France's Christ-Emmanuel Maouassa during the qualifying match of European U21 Championships 2021 between Georgia and France in Gori, Georgia, Sept 4, 2020. (Xinhua/Kulumbegashvili Tamuna)
"The sport venues are almost completely finished. All the constructions are on time. The financing for these Games, and especially the private contributions from the Chinese business players, has been unprecedented," he said.
In a lifeless first half between two newly promoted sides, Ever Bright striker Luiz Muriqui pulled muscle and was substituted in the 24th minute.
The women's long jump event saw teenager Qin Huiling better her PB by three times to land the victory at 6.37m. Zeng Rui also renewed her life-time best to win the women's triple jump title at 14.10m, while Fang Yaoqing won the men's triple jump at 16.88m. Enditem
He became a first-team regular under manager Renato Gaucho following the departure of Everton Soares to Benfica last month.
R&F broke the deadlock while Saba forced Jianye defender Chen Hao into an own goal in the 12th minute.