Tanking is a strong accusation to make in any sport but, no doubt, Jack Sock lacked the heart expected from a defending ASB Classic champion.

Losing in straight sets in his first match could, perhaps, be forgiven if Sock played to his potential. Had he genuinely put in for the full match; had he given everything to the tournament that first gave him a wildcard back in 2014 when he was first emerging.

That was not the case as he meekly surrendered 6-3 6-3 in 62 minutes to unheralded German Peter Gojowczyk, ranked 65th in the world. This was his second victory over a top 10 opponent and set up a quarterfinal showdown with Dutch veteran Robin Haase.

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Sock, comparatively, looked nothing like the world No.8.

Up a break in the first set was as good as it got. From there, Sock looked unfocused and, as the match wore on, grew increasingly disinterested.

He offered excuses about a shorter offseason than planned; a hip injury that limited him to two-half-matches last week but then in the same breath said he was fully fit.

Rust is a genuine concern for players starting the season in Auckland. But in his three previous trips here Sock enjoyed a 10-2 record which included two appearances in the final.

In a testy post-match press conference Sock was defensive about his on court attitude; the way he went big on second serves late in the match and attempted one unnecessary shot between his legs when in trouble on the scoreboard.

"Am I the first top 10 guy to lose to a guy outside the top 50? Probably not," Sock, who joined fellow Americans Sam Querrey and Steve Johnson on the way out, said. "It was my first match of the year I don't know what you guys really expect of me. Ranking wise I should win but it's not going to happen every day of the year. I'm sorry if you guys thought I was not 100 per cent but I did try.

"It's part of my game plan to serve big second serve I've done that since I was 18-years-old.

"I didn't go into this week saying 'I'm not going to defend my title'. I've won a couple of titles now I know what it tastes like there is no better feeling on tour. I'm not entering tournaments to try make quarters or something. I'm looking to win every tournament I play.

"It's not like a played terrible and he played some amazing tennis on a quicker court with windy conditions. He was slapping balls around and taking it to me."

Sock also lost his first round doubles match on Tuesday, and therefore departs for Melbourne to prepare for next week's Australian Open. He is keen to return next year, but tournament director Karl Budge may think twice if this is the effort he gets.

It was not a return on investment.

"I'm going to turn around and get some preparation now I'm not here as long and see what I can make of it.

"I love Auckland the fans are incredible here. I love the court. I consider myself a loyal guy I'm still here playing every year and still loving the event. It's been great prep for me but not my day, not my week."