Torstein Tren completed his first day in the Tour de France yellow jersey on Wednesday's fifth stage, although it wasn't without incident and tomorrow offers its own challenges too.The peloton tackles its first hors-category mountain on stage six in the form of the Col du Tourmalet, which itself is sandwiched in-between the cat-one Col d'Aspin and the cat-two finishing climb to Gavarnie-Gdre. In theory, his 7:53 lead over Tadej Pogaar (UAE Team Emirates-XRG) and Jonas Vingegaard (Visma-Lease a Bike), should enable Tren to hold on to the yellow jersey. But, speaking looking ahead to it on Wednesday evening, he said "Tadej is Tadej" and nothing was guaranteed."We have to see how fast they're going. And of course, Tadej is Tadej, and I think if he goes full gas on the Tourmalet, maybe I will be behind. You don't know how much you will be losing. We've just got to see tomorrow."The high temperatures, which cooled mid-week to a balmy 31deg C (88F) on stage five, are due to pick up again, which made things even less predictable, he said."We haven't had a lot of real mountain climbing yet," he added. "So we don't know what will happen, and then with this heat everything can happen, so you just have to hope and do your best."The 30-year-old, who is a former Tour de Suisse stage winner and wore the leader's jersey at the Vuelta a Espaa last year, came off his bike at one point on stage five in an incident he said he didn't know much about. "Suddenly I was on the ground," he said, adding that he was quickly paced back to the bunch and suffered "only a small cut on my knee".How did he enjoy his first ever day in yellow, he was asked. "Yeah, quite nice," came the understated reply. "All the boys said I looked really well, really well. So, yeah, it was nice to have compliments from the boys."When you get cancer, you don't know what will happen," he added, referencing his 2022 testicular cancer diagnosis (which was successfully treated), "and I'm obviously quite happy to be back at a good level and leading the biggest race of the world."