Jack Sock has been no lock

Bound for tennis glory.

A chip off Andy Roddick’s block?

Or just a wishful story?

But wait. Sock, the fellow linked at an early age to the last American male major-tournament champion, is still only 23. And Friday, he made something of a statement about big-time future — possibly the near future — success.

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Repeatedly flashing his eponymous shot, a socked forehand, he powered to a thorough 6-4, 6-3, 6-3 third-round U.S. Open upset over Marin Cilic, the tournament’s 2014 champion.

Not only was that a shock to the seeding order — Sock No. 26, Cilic No. 7 — but also a boost to the satirical “Jack Sock for President” ad campaign by Sock’s racket company, Babolat. (The campaign slogan is “Let’s Rally America 2016.”)

“It can only help my campaign,” Sock agreed. “It’s been a good year. I think I’m the only one running, so I’m in a good position.”

A fifth-year pro, Sock didn’t break into the world rankings’ top 25 until November. But since then he has reinforced some old expectations that had been fostered by his 80-0 match record as a Kansas high schooler and as the 2010 U.S. Open junior champion.

In July, Sock rallied from an 0-2 set deficit to beat Cilic in five during the U.S.-Croatia Davis Cup tie. Last month, after being eliminated in the Rio Olympics’ singles, Sock paired with Bethanie Mattek-Sands to win the mixed doubles gold and with Steve Johnson for the doubles bronze.

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Cilic, too, arrived in Flushing Meadows on a roll, having won the pre-Open tuneup in Mason, Ohio.

But after he and Sock routinely held serve through the match’s first six games — all concluded with short points — Sock pressured Cilic into a netted forehand to break ahead, 4-3. In full control the rest of the way, Sock never even allowed Cilic the glimpse of a break point.

“I was not able to get into the rallies,” Cilic said. “All credit to him; he was pretty comfortable on his serving games and I was under a little more pressure.”

Maybe, Sock said, a new outlook had something to do with his efficiency. “Maybe in the past years I was happy to make the third round and, you know, kind of whatever happens, happens,” he said. “I feel definitely more on a mission this year.”

In 2011, when Sock made his debut in the Open’s main draw against Andy Roddick, much was made of his Roddick-like potential and their common Nebraska roots — “Maybe we need more cornfed boys,” Roddick said then, though both grew up in other states.

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But it took two more years for Sock to break into the top 100, and another year to stay there, while tennis seers went looking for other young players to be the Next American Champ.

So now, back to Sock? Is this the breakthrough? “I think it’s a buildup,” Cilic said. “You have to be on a good level of performances during the year. It’s not just on and off, especially in the big tournaments. That’s where it counts. It means more.”

To celebrate this new meaning, Sock set about “fencing with my racket” in his postmatch celebration. “I became good buddies in Rio with one of the fencers, Miles Chamley-Watson. He lives here in New York, and I’ve been trying to get him to come out and watch a match.

“He came out today. Kind of on the spot, I thought of turning the racket into — I think it’s called a foil?”

A foil? That would be what Sock was to Cilic.