Jonathan Rea won race one at Donington Park to move into the World Superbike Championship lead by nine points as rival Alvaro Bautista crashed out.
The Northern Irishman, aiming for a fifth straight world title, jumped pole-sitter Tom Sykes off the line and sprinted clear to win by 11.3 seconds.
Bautista was sitting in fifth place before crashing with 13 laps to go.
Rea's victory in wet conditions means there has been a 70-point swing in the standings in just five races.
Kawasaki rider Rea, 32, got the perfect start from second on the grid and had a five-second lead by the end of lap two.
The four-time champion survived a scare after running wide with 10 laps to go, but held on to win, ahead of BMW rider Sykes and Kawasaki team-mate Leon Haslam, and went top of the standings for the first time this season.
Bautista lost the rear end of his Ducati coming out of Starkey's Bridge on lap nine - and it was the second time during this campaign that he has failed to finish.
"It's mad," Rea told Eurosport after the win, "Sometimes you feel like the underdog when you come from behind but I knew I couldn't throw points away, so I was really nervous.
"It was the longest race of my life in those conditions. I had a couple of moments but my bike was incredible, I'm super happy."
Isle of Man TT lap-record holder Peter Hickman impressed after his late BMW call-up, finishing in seventh after qualifying in fourth position.
Eugene Laverty's return from injury was put on hold, with the Toome rider forced to withdraw from his home championship round after second practice.
Rea will start at the front for Sunday's Superpole race, with another feature race taking place to round out the weekend.
|2019 WSBK Riders' Championship|
|Jonathan Rea (GB)||Kawasaki||339 pts|
|Alvaro Bautista (Spa)||Ducati||330|
|Michael van der Mark (Ned)||Yamaha||196|
|Alex Lowes (GB)||Yamaha||175|
|Leon Haslam (GB)||Kawasaki||169|
|Tom Sykes (GB)||BMW||130|