Karolina Pliskova is struggling under the pressure of trying to keep the world No 1 ranking and repeat a trip to her first Grand Slam final at the US Open.

The 25-year-old Czech top seed, last year’s runner-up on the New York hardcourts, saved a match point with a forehand winner in the 11th game of the second set Saturday in rallying to defeat Chinese 27th seed Zhang Shuai 3-6, 7-5, 6-4 and reach the last-16.

“It’s kind of tough for me this year to play here. Never was in this position to defend a final of a major. And I’m first seeded, so it’s like totally different story,” Pliskova said.

“I feel pressure. I know all the opponents are behind me [in ranking] so I should win every match, which is tough.”

Wimbledon champion Garbine Muguruza, also into the fourth round, leads the ranking chase while Ukraine’s fourth-seeded Elina Svitolina needs at least a semi-final run and Pliskova must return to the final to have a chance of keeping the top spot.

“I’m not really fighting just for this position but I just want to play well and I want to win,” Pliskova said.

“Knowing you played fine last year, you just want to play that good again. I don’t feel like I’m playing that good again, but I’m still in the draw so I’m not going to be that sad.”

Muguruza is a two-time Grand Slam champion, having won last year’s French Open and the Wimbledon crown in July. Neither Pliskova nor Svitolina have won a Grand Slam title.

And the top spot was basically vacated this year by Serena Williams, who won her 23rd Grand Slam title while pregnant in January’s Australian Open.

Her baby girl was born on Friday and she hopes to return to defend her title next year in Melbourne.

Pliskova, who complained of right arm tightness during the match, was searching for consistency in her form.

“I’m playing in quite big waves so far,” she said. “Sometimes I think I play good shots, good rallies, and then there is hundreds of mistakes and not a good shot.

“I just try to fight even if my game is not 100 percent and it’s not 100 percent actually this week. But I’m winning, so that’s the main thing.”

Still, she’d prefer not to face match point Monday when she faces American Jen Brady for a quarter-final berth.

“I don’t want to get in this position that she’s serving for the match, actually,” Pliskova said. “Maybe next time I should play a little bit better earlier.”

Svitolina beat 62nd-ranked American Shelby Rogers 6-4, 7-5, to sustain her chances at swiping the top spot.

The five-time WTA 2017 champion gave an apologetic shrug on match point, a net cord winner just plopping across.

“I was just trying to earn my moment, to fight for every ball. The last ball I was very lucky,” she said. “Hopefully I can go further.”