Shortly before the finish line, the Slovenian, who started the day 1.34 behind overall leader Remco Evenepoel, pulled away and brought four other cyclists with him.
He was in the lead towards the finish in Tomares after 189 km in Spain around and was passed.
As he rolled to get into a new position to sprint, he caught the handlebars of Fred Wright and crashed badly with his clothes scraped and blood white, just 50 meters from the finish line.
He was helped by a stable mate in Jumbo Visma to laboriously cross the finish line.
Then a shattered Roglic, chasing his fourth straight overall victory, sat devastated and recovered.
There were painful images on Eurosport.
Took eight seconds off the leader
Despite the crash, he gained eight seconds on Evenepoel, who punctured in the last three kilometers, which meant he got the same time as those he was riding with at the time.
- I heard that Primoz crashed and really hope that he is okay and can continue the race, says Evenepoel according to Reuters.
The Dane Mads Pedersen, followed up the country's recent successes, and won the stage clearly after a perfectly staged sprint.
The competition is part of the Grand Tours, the others being the Giro d'Italia and the Tour de France, where 21 stages are run.
It's the finish line in Madrid on Sunday.