Europa League competitors Stuttgart went into the Bundesliga winter break in ninth place after three wins in the last four games to stay in contention for another European spot next season.
"We are very happy that we managed to sign Gotoku Sakai about a year ago," said Stuttgart sports director Fredi Bobic. "We knew he had enormous potential in his position. He has developed in a great way since."
The two new deals follow contract extensions for Niedermeier as well as midfielders Tamas Hajnal and Christian Gentner this week.
The Bundesliga resumes next week after a one-month break.