Germany coach Joachim Low gave his players a day's rest on Thursday, amid fitness concerns for defender Jerome Boateng.
The world champions need to get through four more matches to claim their fourth European Championship title -- and first since 1996. It starts with the round-of-16 game against Slovakia in Lille on Sunday.
Boateng is bidding to overcome the calf muscle problem that forced his early substitution in the 1-0 win over Northern Ireland on Tuesday.
"The medical team and Jerome are working hard on it. We're all very positive about it and hope it works out. We'll do everything to make sure [he's fit]," Germany assistant coach Marcus Sorg said on Thursday at the team's tournament base.
Though Sorg fell short of giving a definitive answer on whether the 27-year-old defender, arguably Germany's player of the tournament so far, will be fit by Sunday, saying: "I'm not a prophet."
If Boateng does not regain fitness in time, Benedikt Howedes or Shkodran Mustafi will likely partner Mats Hummels in front of goalkeeper Manuel Neuer.
"We have perhaps two of the best three central defenders in the world in Mats and Jerome. But we've a lot of trust in the guys behind them, in Beni and Musti," assistant coach Thomas Schneider said. "We trust the players that come off the bench."
Slovakia defeated Germany 3-1 in a tournament warm-up game in Augsburg on May 29 thanks to goals from Marek Hamsik, Michal Duris and Juraj Kucka.
However, Germany were missing first team regulars Neuer, Hummels, Toni Kroos, Thomas Mueller and Mesut Ozil, on a water-logged field for the second half, thanks to a storm that interrupted the match.
"We can't take the friendly game as an indicator," Schneider said. "We know that Slovakia asked a few questions at the time but we were in preparation mode. Now we're in the knockout rounds. There are different factors at play."
Slovakia finished third in Group B behind Wales and England, its 2-1 win over Russia in Lille proving crucial to its progression before it sealed the deal in the scoreless draw with England.
"They're a technical side, very direct and compact, and they've shown their defensive side already in the tournament," Schneider said. "For us it's a hard task but a task we can manage and one we definitely will."
Low, who had earlier said his side knew what to expect against Slovakia, warned against any complacency.
"We want to stay as long as possible in France. The smallest mistakes now decide whether we go home or reach the next round," said Loew, who gave his players their second day off in the tournament to stay fresh and recover from their travels.
Germany's team base is about 500 kilometers from Paris and more than 600 from Lille, cities where the side played all its group games.
If Germany defeats Slovakia on its return to Lille, the team will play either Spain or Italy in Bordeaux, where progression would mean a semifinal match in Marseille. The final is back at Stade de France near Paris.
"All the concentration is on the next game and then we'll see what happens afterward," Schneider said.
"It's not bad that Italy are playing Spain as one top teams will be out. In the end you have to beat every team if you want to be European champion."