The first flat stage from Lorient to Pontivy looked set to be pocket-rocket Ewan's time to shine after taking 17 points in the intermediate sprints through the previous two hilly stages to put himself in a great position to challenge for the green jersey.
And up until the finish it was business as usual, with Ewan was again first in the peloton through the intermediate sprint in stage 3, taking another 11 points as he lined himself up at the helm of the pack heading into the finale.
But tragedy struck in the final sprint, as the 26-year-old found himself boxed in with little room to manoeuvre, clipping his wheel and colliding with Sagan as the lead group rounded the last bend.
The pair came down hard and slid to a stop on the concrete as Tim Merlier (Alpecin-Fenix) rode home for the stage win.
Ewan looked to be in significant pain following the crash, but managed to cross the finish line and was immediately taken to hospital for treatment with Lotto Soudal later confirming the extent of his injuries and withdrawal from the race.
Recounting the crash, Ewan revealed he planned to attack early in the sprint but was forced to change tack to avoid the other riders which ultimately caused him to clip the wheel of Sagan.
"It all happened quite quickly," Ewan said. "I just remember I wanted to go quite early in the chicane and I started sprinting on the left, but saw the guys at the front were closing to the right, so I had to stop sprinting and hope for it to open up again.
"When all that happened I came next to Peter [Sagan] and we were quite close together on the wheel.
"Then when Merlier went again to the right I just touched the wheel and went down."
Ewan said he could tell straight away there was something wrong and confirmed the serious extent of the injury but was optimistic about the prospect of future recovery.
"Usually when you crash, you don't feel so much because of the adrenaline," he said.
"But straight away I felt a lot of pain, they were pressing on my collarbone and I could feel it clicking.
"It's the first bone I've ever broken, they told me it's broken in four spots and I have to get surgery on it to put it back in place.
"I think out of any bone to be broken it's one of the easiest to come back from, recovery-wise."
Whilst the injury is a huge setback in his career, it appears Ewan has come to terms with it as part and parcel of the sport and indicated his desire to compete at the Vuelta a España in August if he can recover in time.
"This is part of cycling unfortunately," he said.
"I just have to let this heal and when I can train again, I'll start training and speak with the team and look towards the future for what races are going to suit me the best.
"I think it's seven or six and a half weeks til the Vuelta, so there should be plenty of time to rest and heal and train and hopefully that can still be a goal of mine this year."
Fellow Aussie Jack Haig (Bahrain Victorious) was also forced to withdraw from the Tour after suffering a collarbone fracture and concussion in a crash with five kilometres to go.
The Tour de France continues tonight with Stage 4 from Redon to Fougeres and you can catch all the action LIVE on SBS and SBS On Demand from 9:30pm (AEST), with SKODA Tour Tracker App starting from 9:15pm (AEST).