Kastelijn escapes to win longest stage of Women's Tour de France
Kastelijn was part of a 14-rider breakaway on the 177km stage, the longest of this year's race.
The Dutchwoman surged away on a climb and had pulled a minute clear by the finish, giving the Fenix-Deceuninck team, in its debut top-level season, a first victory on the Tour.
"The final metres were amazing," she told broadcaster Eurosport.
She was followed home by three other Dutch riders.
Demi Vollering was second after a ferocious battle over the closing kilometres.
Anouska Koster was one second behind and defending champion Annemiek van Vleuten was fourth, another second back.
Belgian Kopecky, who attacked first from the peloton, faded to 14th, 16sec behind Vollering.
"It was a crazy day," said Vollering, who despite gaining bonus seconds was unhappy she had not been able shake off her rivals.
"Normally we don't have such long races. You really feel it in the bunch, everbody's really tired and no one has that really explosive attack to go away, but everyone can follow."
Vollering jumped to second overall, 43 seconds behind Kopecky. Van Vleuten, rose to sixth overall, 51 seconds off the lead.
On Thursday, the 126-kilometre stage from Orne-le-Chateau to Albi will favour the sprinters.
The Women's Tour de France finishes on Sunday with a 22km time trial in Pau, the day after the queen stage with the finish at the summit of the Tourmalet.
