Auckland - Top seed Jack Sock rejected suggestions he may not have been trying as he crashed out of the ATP Auckland Classic on Wednesday to unheralded German Peter Gojowczyk.
The American made a strong start with an early service break but then folded as Gojowczyk took control to beat the world number eight 6-3, 6-3 in just over an hour.
Sock's fellow American seeded players Sam Querrey and John Isner also exited in the second round but put up more of a fight.
Third-seed Querrey lost a marathon battle to Czech Jiri Vesely 6-4, 6-7 (10/12), 7-6 (7/5) while fourth-seed Isner fell to 21-year-old Korean Chung Hyeon 7-6 (7/3), 5-7, 6-2.
Defending champion Sock had a bye in the first round and dismissed his lack of form by saying he was "rusty" and claimed he had tried his best against the German despite some odd shot selections.
"It's my first match of the year, I don't know what you guys expect of me," he said when questioned about his approach to the game.
While it was Sock's first ATP tournament of 2018, he did play in the Hopman Cup last week where he was narrowly beaten by Roger Federer 7-6, 7-5.
Sock said credit also had to go to Gojowczyk who is ranked 65 in the world.
"I was up a break in the first set, you guys act like I lost 6-0 6-0," the 25-year-old American said.
"The guy hit some unbelievable returns, he was painting lines all over the place, slapping balls coming in ... It was a rusty first match, but you've got to get the cobwebs off, the ball didn't feel as good as it has in the past."
Second seed Juan Martin del Potro had no trouble disposing of rising Canadian Denis Shapovalov 6-2, 6-4.
Fifth seed and 2016 champion Roberto Bautista Agut came from a set down to beat Steve Johnson 2-6, 6-2, 6-1, while four-time champion David Ferrer was untroubled in his 6-2, 6-2 win over Joao Sousa.
In the quarter-finals, Vesely plays Bautista Agut, Gojowczyk faces Robin Haase, Ferrer plays Chung and del Potro clashes with Karen Khachanov.
Results from day three of the ATP Auckland Classic tennis tournament on Wednesday (x denotes seed):
Roberto Bautista Agut (ESP x5) bt Steve Johnson (USA) 2-6, 6-2, 6-1
Peter Gojowczyk (GER) bt Jack Sock (USA x1) 6-3, 6-3
David Ferrer (ESP x7) bt Joao Sousa (POR) 6-2, 6-2
Juan Martin del Potro (ARG x2) bt Denis Shapovalov (CAN) 6-2, 6-4
Hyeon Chung (KOR) bt John Isner (USA x4) 7-6 (7/3), 5-7, 6-2
Jiri Vesely (CZE) bt Sam Querrey (USA x3) 6-4, 6-7 (10/12), 7-6 (7/5)
Karen Khachanov (RUS) bt Pablo Cuevas (URU x6) 6-2, 7-6 (7/4)
Robin Haase (NED) bt Lukas Lacko (SVK) 7-6 (7/4), 6-3