Mena struck in the 41st and 51st minutes at Estadio Victoria as the visitors secured their fifth win from eight matches.
Awaiting him in the next round is 20th seed Pablo Carreno Busta of Spain.
Germany took the reins and pressed forward whereas Spain looked shocked and unable to respond.
"We do not know which contract you have analyzed and what is the basis for you to conclude that it has a "recession clause" which is applicable if the player decides to unilaterally extend this contract."
Both sides had chances to break the deadlock in the first half but missed them all.
It's the second straight victory for Portugal, as they defeated Croatia 4-1 at home last Saturday. They will visit France in October in their third match in Group 3.
The Tricolor failed to qualify for the 2018 World Cup in Russia and were dumped out of last year's Copa America in Brazil before the knockout phase.
The Dutch world 1,500m and 10,000m champion moved ahead once more in the final minute and successfully maintained the lead until the hour elapsed to reach 18,930 meters, smashing the previous mark of 18,517 meters set by Ethiopia's Dire Tune in 2008.
"All the people involved with the Games are working together to prepare, and the athletes are also making considerable efforts toward next year under the circumstances they've been handed," Hashimoto said during a press conference.
Elsewhere, Belgium's Elise Mertens upset second seed Sofia Kenin of the United States 6-3, 6-3 to also make the last eight.
Bennett became the sixth Irishman to win a stage at the Tour and ensured he now has stage wins at all three of cycling's Grand Tours. He won three at the Giro d'Italia in 2018 and two at the Vuelta a Espana in 2019.