It sounds fishy.

Defending U.S. Open champion Sloane Stephens survived a major second-round scare Wednesday against unknown qualifier Anhelena Kalinina — aided by a mid-match snack of sushi.

On another brutally hot and sticky day, the 24-year-old American lost the first set but rallied to defeat the 134th-ranked Ukranian, 4-6, 7-5, 6-2, amid the 96-degree conditions of Flushing Meadows.

After splitting the first two sets, the players took their extreme-heat-policy 10-minute break in the locker rooms, where Stephens revealed she sat on the floor and had a plate of sushi — the well-known energizer.

“I just needed to eat something,’’ Stephens said. “That was what was upstairs and they brought me. That’s what I had to eat. There’s no secret behind it. It’s just what was available.’’

Well it worked, as she saved her best tennis for the final stanza to set up a difficult third-round match against Victoria Azarenka, a two-time Grand Slam champion with a few more credentials than Wednesday’s opponent.

“Just happy to be through,’’ Stephens said. “I was proud. I came out bouncing in the third set.”

However, it bears watching what toll this nearly three-hour match takes on Stephens as she looks to defend the title after shakily moving to 9-1 against qualifiers. Even the third set was no picnic. She saved two break points to avoid going down 2-0.

“A tough day, obviously,’’ Stephens said. “I just found a way and sometimes it’s not going to be your best. I had to weather the storm.’’

It’s too bad this one didn’t come more quickly and easily, as Stephens couldn’t pick her second-round opponent out of a lineup.

“I literally never seen her before so I was like, oh OK,’’ Stephens said. “I was like, OK, that’s who that is.’’

Kalinina had never been in a Grand Slam main draw before this Open and hadn’t faced a player ranked better than 49th in her career.

It’s been a rocky Grand Slam season for Stephens following her stunning Open breakthrough last September. Stephens flamed out in the first round of both the Australian Open and Wimbledon this year.

Stephens, who complained about loud conditions after her opening match in the brand-new Louis Armstrong Stadium, on Wednesday was back at Arthur Ashe Stadium, where she won last year’s crown. Stephens-Azarenka could be in Ashe Stadium again.

“She’s playing with a bit of confidence and so am I,’’ Stephens said.

She’s also playing with blisters, contracting three in the past three weeks, blaming the hot weather. She showed inconsistency early on — especially with her first serve.

“I’ve never had a blister, ever,’’ Stephens said.

In the first set, Stephens fell behind a break at 4-1 and Kalinina pummeled a backhand down the line to set up another break point to go up 5-1. Stephens survived and got the match back on serve at 4-4. But at 4-5, Kalinina broke Stephens to win the set. On a third set point, Kalinina ripped a Stephens second serve that the American flailed at and it sailed deep.

“She was playing really well, playing four matches already with [qualifications],’’ Stephens said. “She got used to the conditions.’’

While she waits, Stephens is laying low in Manhattan. She was headed to dinner (sushi, perhaps?) with a friend Tuesday night — five blocks from the hotel. After walking a block, Stephens tanked.

“We got to the corner I was like, ‘No, no, no, we’re going back,’ ’’ Stephens said. “I’m not walking five blocks.’’