Ferrer, seeded No. 1 and going for a fourth consecutive title in Auckland, defeated Guillermo Garcia-Lopez 6-3, 5-7, 7-4 by going 4-for-4 in break chances. It was Ferrer's 14th consecutive win at the event.
Garcia-Lopez won a solid 62 percent of the points on serve but Ferrer was able to finish off all of his break opportunities, including two in the third set.
Isner had a typically server-dominated match with fifth-seeded Phillip Kohlschreiber and emerged with a 6-7 (4-7), 7-6 (7-4), 7-6 (7-5) win in 2 hours, 24 minutes. The two combined for 26 aces -- 18 by Isner -- and saved a total of 10 break points -- seven by Kohlschreiber -- leaving the match to be decided in the series of tiebreakers.
Isner won 82 percent of the points on serve and Kohlschreiber 72 percent.
Also Thursday, Lu Yen-Hsun ousted Steve Johnson 6-4, 6-4 by saving four of the five break points he faced and Roberto Bautista Agut stopped Jack Sock 6-1, 6-3 by taking all six of the break chances Sock had.
Ferrer goes against Lu in Friday's semifinals. Ferrer has won two of the three times he's played Lu, including in the second round last year in Auckland. The first semifinal sends Isner against Bautista Agut. Isner beat Bautista Agut in three sets in Beijing last year in their only previous meeting.