Kasia Niewiadoma faces what she called the “most important day of her career” in the French Alps as she bids to contain Demi Vollering and take overall victory in the Tour de France Femmes on Sunday.

The growing rivalry between Niewiadoma, the race leader, and Vollering, the defending champion, will reach a head on a gruelling mountain stage that features two mighty climbs, the Col du Glandon and the famed ascent of Alpe d’Huez.

Related: Justine Ghekiere bursts clear to wins stage seven of Tour de France Femmes – live reaction

The Polish rider accepted it was make or break for her hopes of success and for her Canyon/SRAM team. “It’s the most important day of my career and maybe my team’s existence,” she said. “But we’re ready for it and we’re very happy to still be in yellow. Everyone is very motivated and dialled in. We’re here to pursue our dreams.”

There are clear signs that Vollering, who crashed on Thursday’s stage to Amnéville, is building up to a major assault on Niewiadoma. “I wanted to make a difference today, but the climbs were not steep enough,” she said. “Tomorrow, on the Alpe, will be different.”

The duo had differing perspectives on the denouement of the seventh stage when the race leader attacked, only to be chased down by her rival and then dropped in the final 200m. It was enough for Vollering to snatch a four-second bonus and regain some morale, with the 21 hairpins to the Alpe to come.

The race favourite, whose challenge looked to be hanging by a thread after her crash at the end of Thursday’s stage, appeared to be getting back to her best. “Kasia seemed very nervous, looking behind her a lot,” the Dutchwoman said. “Getting those extra seconds at the finish was a nice feeling, but tomorrow, with the real climbs, it should be enough for  me.”

Niewiadoma, though, was dismissive of that notion. “I lost four seconds, so that’s nothing,” she said, adding that others in the peloton also posed a threat.

“I don’t only focus on Demi,” the 29-year-old said. “I see a lot of good riders in the peloton. Every team has somebody who can play a ‘gold card’ in tomorrow’s stage.”

The Tour’s first real mountain stage, at 166.4km the longest of the race, was won by Justine Ghekiere, who rode alone to the finish from a six-rider breakaway that disintegrated on the approach to the final climb of Le Chinaillon.

The Tour’s longest day took the peloton from the northern Jura to the Haute-Savoie over five categorised climbs, including the first-category Col de la Croix de la Serra and the second-category finishing climb, Le Chinaillon, above Le Grand Bornand.

With the favourites seemingly content to mark each other until the last climb, Marianne Vos reignited the fuse for the Netherlands, becoming race leader on the road by some distance.

The break, which also featured Ghekiere and her teammate Julie Van de Velde, Sara Martín, Ruth Edwards and Sarah Roy, was still well ahead of the peloton as it entered the final 20km. But on the long haul up the penultimate climb, the Col de Saint-Jean-de-Sixt, the group fell apart and Ghekiere, the Queen of the Mountains leader, ­accelerated clear on her own.

With Niewiadoma’s Canyon/SRAM team and Vollering’s SD Worx Protime picking up speed, Vos’s threat to the overall standings was extinguished 3km from the finish.

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“I was getting a lot of support from my ladies, which allowed me to save energy for the final,” Niewiadoma said. “It was super long and very challenging from the start, so I was happy with how the legs felt at the end. We didn’t lose anything and we’re still up there, 24 hours to go.”

But the back-to-back climbs of the Glandon and the Alpe are another challenge altogether. “The Alpe is going to be a very personal effort,” she said. “Everyone will have to dig deep because it’s very steep and long. I just want to make sure I arrive at the bottom of Alpe d’Huez relatively fresh, so I can give my best.”