Cortina (Movistar) third ahead of two teammates.
Girmay is the first race leader of the Volta a la Comunitat Valenciana, the first time he has led a European stage race.
Thursday’s 178km second stage is another hilly day in the saddle between Novalda and Alto de Pinos, again near Calpe.
Girmay made his 2023 season debut at the Challenge Mallorca, taking third and then second in two races before his win on the Spanish mainland.
“I’ve been waiting for this victory for almost a month,” Girmay explained.
“We worked together in the training camp, and I'm in quite good shape now. I was close two times in Mallorca, and now to find the victory was amazing, I’m super happy. Thanks to my team for what they did today. They did a great job."
Girmay leads Kooij by four seconds due to the time bonuses awarded at the stage finish but played down any overall ambitions due to the hilly nature of the Volta a la Comunitat Valenciana.
“I don’t think so. My plan was to come here and win a stage. The plan is not to go for GC but to perhaps win another stage. We’ve also got Rui Costa here, and we hope he can go for GC.”
The 189km stage started with a long ride along the Mediterranean coast via Alicante and Benidorm before climbing into the hills that teams and riders so often cover during winter training camps.
Mikkel Zberg did not start, leaving just six UAE Team Emirates riders in the race, with the USA’s Brandon McNulty the protected team leader after Juan Ayuso was unable to start due to tendinitis.
Five riders formed the early break, but Jumbo-Visma and then Intermarché-Circus-Wanty lead the chase, first holding them at 3:00 and then reeling them in. Xabier Isasa (Euskaltel-Euskadi) won the intermediate sprint, but the attack soon fell apart as the hills began in the second half of the stage.
Eddie Dunbar was caught in a crash mid-stage as he made his debut with Jayco Alula but was quickly up again as Bahrain Victorious, Bora-Hansgrohe, Ineos Grenadiers and Intermarché-Circus-Wanty all made sure they were well placed as the Puerto de Rates climb started.
Marc Soler (UAE Team Emirates) was first to the top as Alexander Kristoff (Uno-X), and Kooij were dropped. However, the young Dutch sprinter managed to get back on while Kristoff was forced to chase.
Bahrain Victorious pushed the pace on the descent and again on the lower slopes of the Alto de Bernia climb. They were looking to distance some of the fast finishers and set up Fred Wright, but Girmay proved difficult to drop.
Gino Mäder led Bahrain Victorious over the top of the Alto de Bernia and also drove down the descent. The Kristoff chase group was at 45 seconds, so Jumbo-Visma drove the front group to the finish. They controlled the lone attack Joan Buo (Euskaltel-Euskadi) and then tried to set up Kooij.
However, Intermarché-Circus-Wanty also had a lead-out train and protected Girmay until he opted to do his own thing along the barriers.
Girmay struggled to see the finish line but sensed it was coming and opened up his impressive sprint.