In the opening laps in Portimao Sainz was able to take advantage of a track made slippery by rain, and also by the fact that he was on soft tyres.
Three cars that qualified ahead of him – the Mercedes duo of Lewis Hamilton and Valtteri Bottas and the Ferrari of Charles Leclerc – started on the medium tyre.
From seventh on the grid Sainz was soon up to fifth, and he then gained a spot when Sergio Perez spun. He then overtook Max Verstappen and Hamilton to claim second by the end of the first lap, before passing Bottas for the lead on the second lap.
He remained in front until Bottas re-passed him on lap six, while his teammate Lando Norris also ran as high as fourth, before both men slipped down the order.
Sainz eventually finished sixth, while Norris was out of the points in 13th after a costly tangle with Lance Stroll.
"Obviously it gave some nice pictures," said Seidl when asked by Motorsport.com about Sainz's spell in front.
"Which we will put up at home for additional motivation in order to keep us going towards the goal we all have to get back to the front end in F1 in the coming years.
"So that gave a nice taste, and it was good to see, to have some leading laps. But at the same time it was also clear that we had to focus on our own race, or the race which we were in. And so we were quickly back then in the fight where we belong.
"But was interesting to see. We know from Carlos and Lando, they like these conditions also. Of course, we had an advantage with the soft tyres compared to the guys on the medium tyres, to Mercedes and Charles.
"But compared to Verstappen who was also on soft tyres, Carlos put up a good show, and could actually put in a nice gap within some laps. But then we were quickly back to reality."
Tyre graining proved to be a handicap for the Woking team in Portimao.
"With Carlos it was a tricky race because it was also not easy on the strategy side to get it right," said Seidl.
"I think we did a good job in order to outscore both Racing Point and the Renaults, our direct competitors in the constructors' championship, with I think also a car that was quicker in the race than these guys.
"That's positive. In the end we suffered with graining, especially on the soft tyres at the beginning. Which then simply didn't allow us to fight higher up. Carlos did a great job, also with his overtaking moves, to hang in and then got the Renaults and Checo in the end, which was positive for us."
Seidl says the latest updates are continuing to bring performance to the car.
"Overall, I was quite happy with how the weekend went after Nurburgring," he noted. "I think we started with a solid car configuration with the new nosebox and aero concept on Friday. We tested some more updates during Friday, which we plan to introduce from Imola onwards, after seeing the positive results.
"And then also for the rest of the weekend, I think we had a competitive car, which was good, which was another confirmation that we have a car to fight for this P3 in the championship.
"But in the end, it will be down to reliability, to maximising the races and making sure you see the chequered flag with both cars. This will decide who in the end will finish in P3."