Defending champion Kei Nishikori overcame a slow start to topple Benjamin Becker at the US National Indoor Tennis Championships.
Nishikori had issues with his serve in the first set and Becker broke him twice, but the world No 16 regained his composure to register a 6-4 6-4 victory.
Nishikori, whose third ATP Tour title came at Memphis 12 months ago, will now tackle Alex Bogomolov Jr in the quarter-final.
Bogomolov upset home hope Ryan Harrison 6-3 2-6 6-2 in their second round clash.
Denis Kudla and Michael Russell have also progressed to the last eight after coming through all-American ties.
Kudla, who qualified for the main draw, routed Donald Young 6-1 6-1, while Russell was forced to dig deep for his 6-2 7-6 (7-5) win over Tim Smyczek.
Fellow American Jack Sock was a convincing 6-2 6-1 victor over Jiri Vesely.