For a brief moment, three kilometers from the top of Alpe d’Huez, a chink appeared in Chris Froome‘s armor, and Tom Dumoulin saw it.

Sky’s defending champion had spun up the road in his familiar style, head down, elbows out. It looked like he was on his way to his first Tour de France victory on the famous climb in stage 12. But with a churning tempo behind, Dumoulin chased Froome down, nevermind the fact that he was marked all the way by Sky’s Geraint Thomas, wearing yellow.

Now, he and his Sunweb team director Luke Roberts say they think they’ve cracked the code to beat Team Sky.

“Yeah, I learned they try to keep both in GC and don’t ride behind each other, and Froome had the chance to attack and G [Thomas] didn’t. So, that’s good to know for the future,” Dumoulin said.

Dumoulin is perhaps the only rider with a legitimate chance of challenging either Thomas and Froome in the overall. He is third overall after stage 12, 1:50 behind. Vincenzo Nibali (Bahrain-Merida) is fourth, 2:37 behind, but his future is unclear following a crash that sent him to the hospital after Thursday’s stage.

After that duo of former grand tour winners, the field thins out. Primoz Roglic (LottoNL-Jumbo) is fifth, 2:46 back but completely unproven as a GC leader in three-week races. Romain Bardet (Ag2r La Mondiale) is next, 3:07 behind in sixth but his weakness as a time trial rider could spoil his Tour on stage 20‘s ITT. Plus he has only four teammates left in the race. Movistar’s Nairo Quintana is the only other proven grand tour winner in the top 10, ninth and 4:13 behind after faltering on Alpe d’Huez.

Alpe d'Huez
The group of GC favorites approached the Alpe d’Huez summit finish. Photo: Tim de Waele | Getty Images

So it is up to Dumoulin to challenge Sky. Roberts says that, based on what he saw in the Alps, the possibility to take yellow remains.

“In the last two days they’ve shown their weakness,” said Roberts. “They have a high level with five-six riders. They can ride a hard tempo, but they can’t make Geraint go much faster. Bernal was there and had a good day today. But then Thomas himself was put on the front to pull relatively early. Should they start to feel the effects of the Alps in the Pyrenees, they could show weaknesses.”

Plus, Roberts and the Sunweb team should be encouraged by Dumoulin’s fighting spirit and confidence. After stage 12, the Dutchman fumed.

“I wanted to win today,” Dumoulin said. “I let myself down and made a mistake with shifting in the finale and it was my fault. It was lost before the sprint started. I think I had a chance. Thomas was slightly stronger, but if I played it smart I would have had a start and now it was lost.”

Tom Dumoulin
Dumoulin chased down Froome with Thomas right on his wheel. Photo: Tim de Waele | Getty Images

Dumoulin went on to say he felt he was stronger than Froome on the day and that Thomas sat on his wheel when the wind picked up. “That is the advantage of having two guys in the GC,” he added.

After a flat stage 13 and two medium-mountain stages through the weekend, the Tour reaches the Pyrenees Tuesday. That is where Dumoulin and his Sunweb team will need to determine the best tactic to unseat Team Sky.

“We’ll have to assess after Monday as we head into the Pyrenees,” Roberts added. “Is Thomas still a contender or is he showing weakness into the last week? Do we need to keep the gap close to Froome and then go toward the final TT or do we need to bring some time back on Thomas? We’ll have to see how it pans out.”

Team tactics and missed shifts aside, Dumoulin should be able to count on one thing in the Tour’s second half. Stage 20’s 31-kilometer time trial through French Basque Country will be an ideal day for the world time trial champion to challenge whichever Sky rider is wearing yellow.

Fred Dreier contributed to this report from L’Alpe d’Huez, France.