The German defender is confident that his compatriot will recover in time for the clash, but insists that no one will play if they are not at 100 per cent
The Bayern Munich midfielder, who has started each of Germany's four Euro matches to date, has missed the team's last three practice sessions and admitted on Sunday that he had not yet recovered from an ankle injury.
It was feared that he would be forced to sit out of Germany's semi-final contest against either England or Italy, but Lahm is backing his compatriot to win his race for fitness.
"It is true, having had to miss three training sessions makes things a bit unclear for him," he told reporters at a press conference.
"We need fit players in the semi-finals, we cannot have anyone who is missing three or four per cent.
"Having said that, I do not worry much about Bastian Schweinsteiger's fitness. He knows his body better than anyone else and I am sure he will be on that pitch on Thursday evening."
Germany qualified for the semi-finals in Poland and Ukraine following a 4-2 victory against Greece in the quarter-finals.