South Korea enter their first World Cup quarter-finals clash against a Spain side coach Guus Hiddink knows well but his side are hampered by injuries with little time to recover from their bruising win over Italy.
The co-hosts advanced to the last eight after a stunning 2-1 win over three-time champions Italy on Tuesday.
But they suffered several injuries that could force line-up changes against the favoured Spanish on Saturday, Hiddink said.
The former coach of Real Madrid, he will be up against Raul and other top players from that team.
Hiddink said rest would be vital after the extra time match against Italy in which defender Kim Tae-young sustained a broken nose and defensive midfielder Kim Nam-il twisted his ankle.
"We have a disadvantage regarding the Spanish team, they are already off two days," Hiddink said.
"In these circumstances, playing with such tension, it costs a lot of energy, physical energy as well mental energy."
Hiddink's team showed better fitness than the Italians in extra time but face just their second afternoon clash in these finals, making heat a factor.
"For me, the most important thing is not so much to train but to recover," Hiddink said after a team training session.
South Korea are among the biggest surprises at this World Cup, coming in as underdogs in group D only to finish the group stage ahead of the United States, Portugal and Poland.
South Korea had not won a single World Cup match in five previous appearances spanning 48 years but beat Poland 2-0, dominated before drawing 1-1 with the United States and stunned Portugal 1-0.
After a hesitating first half in which the Italians went up 1-0, Hiddink's team returned to a faster attack seen in the group stage and it again paid off, something Hiddink said they would use against Spain.
"I want them to take the initiative, and then we'll see at the end where we are," he said. "So far, it brought us good results."
Striker Ahn Jung-hwan, who in heading home the golden goal that sent the Italians packing, said he expected Hiddink's experience in Spain to help South Korea prepare.
"I think he knows a lot about the Spanish players and can advise well about how to play," Ahn told reporters at the training session.
"I think if we get some rest and study the strengths and weaknesses of the Spanish players, we can play, win or lose, a wonderful game."