Rovanperä had led since the opening kilometre until a rear right puncture in the darkness of the penultimate Dyfnant speed test cost the 19-year-old Finn 75sec and handed the overnight lead to his Czech colleague.
“It came at the beginning of the stage from nowhere. We were on a big smooth road so I have no idea how it happened,” said Rovanperä, for whom victory this weekend in his Fabia R5 would secure the world title.
Kopecký steered clear of trouble in the rain-soaked forests of north and mid-Wales, and his safety-first policy paid dividends as his other rivals dropped time.
Gus Greensmith held second for most of the afternoon before a water pump problem in his Ford Fiesta R5 ahead of the penultimate speed test left the Briton more than four minutes adrift in third.
Mads Østberg was second at mid-leg service but the Norwegian stopped on the liaison section to SS6 with an oil leak and a lack of power in his Citroën C3. Hayden Paddon’s Fiesta debut ended early when the Kiwi spun and became beached on a bank.
Pierre-Louis Loubet was the dominant force in WRC 2. The Frenchman, driving a Fabia, led all day to end with a 35.7sec lead over veteran former world champion Petter Solberg, a four-time outright winner of Rally GB and competing in his final rally.
Solberg, at the wheel of a Volkswagen Polo, held off Adrien Fourmaux by 3.4sec. Teenage Bolivian Marco Bulacia was a minute off the lead in fourth in another Fabia, with Alberto Heller and Fabio Andolfi completing the top six.
Championship leader Kajetan Kajetanowicz retired his Polo after he twice punctured and had only one spare wheel onboard. Ole Christian Veiby was lying second when a broken track control arm sidelined the Norwegian’s similar car.
Oliver Solberg, who only passed his driving test last week, made an early exit from his WRC debut with broken steering in his Polo.