The 23-year-old Italian is racing in his first Grand Tour – the longest race he did before was the seven-day Tirreno-Adriatico.
Today, after protecting Froome, he took the liberty to sprint. He fell just short, placing second to Italian Matteo Trentin (Quick-Step Floors).
"Today we were there to pull for Chris in the final and last kilometre, I was there and positioned well and thought, 'Why don't I do the sprint?'" Moscon told Cycling Weekly.
Moscon warmed down on his trainer. Sweat from the 34-degree day poured from his face.
"It's too bad I finished second but our goal was just not to lose time and we did that.
"The decision was taken spontaneously. I saw that everyone was positioned well and so I did it and Chris would have liked it and maybe it would have even cost gaps for his rivals. So why not?"
Froome placed seventh in the sprint finish. The group slightly split and caught some off-guard. Trentin's team-mate David De La Cruz (Quick-Step Floors) lost seven seconds and slid from fourth to fifth, losing his place to Wilco Kelderman (Team Sunweb).
"We were riding to stay in front and not to let any time splits go in the final," Froome said. "I didn't want to compete in the sprint myself, but I had Gianni Moscon with me in the final kilometre, he put me in good position and I said, 'Go, try to get the stage' because I could see he was looking strong.
"He came second and that was after all the work he did for me. That's very impressive from Gianni."
Moscon finished fifth in the Paris-Roubaix classic this spring, but is proving just as handy in his first Grand Tour. He guided Froome over the mountains in several stages, including helping in the final of the summit finish to Calar Alto on Wednesday.
"I see that I'm recovering well every day and I hope to be good to support Froome all the way up until the end," Moscon added.
"Today's sprint was very adapted to my characteristics because the last kilometre was demanding. Maybe if I worked a little bit less in the last kilometre I could have had more energy for the sprint but second is OK given the work we did.
"I don't know if I'll have another chance because I'm not sure if there will be another stage like there was today for me. And today worked out well because Quick-Step pulled all day and I was able to save my energy into the last moment. Who knows if this opportunity will happen again."
Sky is already considering Moscon for its Tour de France line up in 2018.
"For his future development we are thinking first of all that he should just stay focused on the classics and try to excel and improve in that area but also bring him to these races and see how he gets on," team boss David Brailsford said.
"We need to see for next year too because you know we can't rule him out of the Grand Tours. You can't rule him out for a Tour de France role next year. You can kind of see already what kind of role he might play in the team."
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