Top seed David Ferrer came from a set down to beat Donald Young and reach the quarter-finals at the Heineken Open in Auckland.
Ferrer lost the first set on a tie-break against American Young but fought back to level and ran away with the decider to win 6-7 (5/7) 6-3 6-1.
Ferrer next meets Spanish compatriot Guillermo Garcia-Lopez, who came through in similar fashion on Wednesday, beating Santiago Giraldo 6-7 (7/9) 6-2 6-3.
The biggest upset of the round was caused by American Jack Sock, who knocked out second seed Tommy Haas 6-4 6-4, while Sock's countryman Steve Johnson posted two victories in a day.
Bad weather had forced two first-round matches to be held over, but that was no problem for Johnson, who defeated Marcos Baghdatis in three sets before seeing off fourth seed Kevin Anderson 7-5 6-4.
American Bradley Klahn completed his first-round victory over ninth seed Daniel Brands but was then defeated 6-7 (3/7) 6-2 6-3 by Lu Yen-hsun.
It was generally a good day for the Americans, though, with third seed John Isner beating qualifier Lukas Lacko.
In the last eight Isner will meet fifth seed Philipp Kohlschreiber, a 6-4 6-2 winner over Horacio Zeballos, while Spain's Roberto Bautista Agut beat sixth seed Benoit Paire 6-3 6-4.