Nishikori won 5-of-8 breaks and served four aces en route to a 6-4, 6-4 win over his German opponent that advanced him to a quarterfinals match-up with Russian Alex Bogomolov.
Bogomolov followed up Tuesday's victory over Sam Querrey with a 6-3, 2-6, 6-2 over American Ryan Harrison.
Americans Michael Russell and Jack Sock also advanced to the quarters Wednesday. Qualifier Denis Kudla of Ukraine joined them with a 6-1, 6-1 win over Donald Young of the United States and will face fourth-seeded Yen-Hsun Lu of Taiwan.
Celebrity Couples of 2014 [PHOTOS]