Croatian Ivo Karlovic pounded 19 aces past second-seeded Feliciano Lopez in beating the Spaniard in straight sets to reach the quarter-finals of the US National Indoor Tennis Championships on Thursday.
Karlovic, ranked 80th in the world, needed 69 minutes to defeat Lopez 7-6 (7/2), 6-4.
While Lopez, runner-up to Japan`s Kei Nishikori in Memphis last year, was denied a return to the final, third-seeded Lleyton Hewitt of Australia and fourth-seeded Lu Yen-hsun of Taiwan both stormed back for three-set victories to reach the last eight.
Hewitt, a former world number one, recovered to defeat Marcos Baghdatis of Cyprus 1-6, 6-2, 6-0.
Lu rallied for a 3-6, 6-3, 6-3 victory over US qualifier Denis Kudla.
Hewitt converted five of his six break chances over the final two sets, wrapping up the victory in one hour and 31 minutes. He has now won three of his five career matches against Baghdatis.
The 32-year-old Aussie reached the Memphis quarter-finals for the third time in four appearances and next faces 35-year-old American Michael Russell.
Hewitt is in search of his second title of the year. He opened the 2014 season with a triumph in Brisbane, where he beat Swiss great Roger Federer in the final.
Lu, 30, set up a quarter-final clash with another American qualifier, Alex Kuznetsov.
Kuznetsov toppled eighth-seeded Mikhail Kukushkin of Kazakhstan 2-6, 6-3, 7-6 (7/1).
Lu will be hoping to improve on his career-best season start. He is 8-2 in 2014 and reached his first ATP Tour final last month in Auckland.
Karlovic will take on rising US talent Jack Sock on Friday, while defending champion Nishikori -- the top seed in an ATP tournament for the first time -- will face American Alex Bogomolov.