Complex sentence: When Harry Blackmun retired in 1994, Stevens became the senior Associate Justice and thus assumed the administrative duties of the Court whenever the post of Chief Justice of the United States was vacant or the Chief Justice was unable to perform his duties.
Split into simpler sentences: When Harry Blackmun retired in 1994, Stevens became the senior Associate Justice. As the senior Associate Justice, Stevens assumed the administrative duties of the court whenever the post of Chief Justice of the United States was vacant or the Chief Justice was unable to perform his duties.