The 27-year-old recently returned to match action after a lengthy spell on the sidelines due to a knee injury and played 59 minutes in his club's 4-1 Champions League final win over Atletico Madrid.
"I am fit, but there are still a few things that I have to work on," Khedira said at a press conference. "I will have to take things one step at a time, though.
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"My objective was to make the World Cup and things have been going very well. I think that I will be in peak condition again by June 16.
"You should always listen to your body, but I'm not holding back in training. I am doing additional exercises next to squad training to be prepared as well as possible. It was a long and hard road to get back. I worked very hard to make the World Cup."
The midfielder then went on to discuss Manuel Neuer's fitness and made it clear Germany is confident the Bayern Munich goalkeeper will also be ready for the game against Portugal.
"We are all convinced Neuer will be top fit in time for the World Cup," the defensive midfielder stated. "He is absolutely determined to make it.
"He is an amazing goalkeeper and was voted best of the world for a good reason. He will be at his best when we meet Portugal."
Germany also meets Ghana and the USA in the group stages this summer.