AUCKLAND, New Zealand -- Top-seeded and defending champion Jack Sock bowed out of the ATP Tour's ASB Classic in its second round Wednesday, beaten 6-3, 6-3 by unseeded German Peter Gojowczyk on a bad day for American players.
Querrey lost 6-4, 6-7 (10), 7-6 (5) to Czech Jiri Vesely, two-time champion Isner lost 7-6 (3), 5-7, 6-2 to Hyeon Chung of South Korea, and Johnson was beaten 2-6, 6-2, 6-1 by fifth-seeded former champion Roberto Bautista Agut of Spain.
Apart from Sock's departure, one of the main surprises Wednesday was the ease in which second-seeded Juan Martin del Potro dismissed Canadian teenage star Denis Shapovalov. Argentina's del Potro, who beat Querrey to win the 2009 ASB Classic, needed only an hour to beat Shapovalov 6-2, 6-4.
Shapovalov beat del Potro when the pair first met in Montreal last year, but the 18-year-old made too many errors Wednesday to repeat that performance.
Isner's powerful serve wasn't enough to save him in his match with Chung, the winner of last year's ATP Next Generation finals.
The 6-foot-10 American, who served 1,123 aces in 2017, hit 30 more Wednesday, but Chung replied with 18 aces and added a superb returning game while Isner made 58 unforced errors.
Johnson, a semifinalist in Auckland last year, looked on target for an easy win when he swept through the first set against Bautista Agut in 35 minutes.
But Bautista Agut stepped up in the second and third sets, putting more first serves in play and returning strongly to create chances at the net.