Fernando Alonso has admitted the improved pace of Mercedes is a "concern" for Aston Martin, with the Silver Arrows having shown a significant upturn in form in Barcelona on Sunday.
Alonso's seventh place finish was his lowest of the season by comparison, with Mercedes' George Russell passing him with relative ease in the first stint as he recovered from a P12 starting slot to take a place on the podium in Spain.
Russell joined team-mate Lewis Hamilton on the podium as Mercedes took 34 points on the day, which saw them leapfrog Aston Martin to second place in the Constructors' Championship in the process.
But with Alonso and Aston Martin having been overtaken on race pace at least around Barcelona, the two-time World Champion admitted his concern about the Mercedes duo as a pairing - highlighting another rival's woes as a reason why Mercedes are forming a partnership to be reckoned with.
"They are fast, they are concern for us. They have been all year long. They have very consistent drivers as well," Alonso said to Sky Sports F1 after the race.
"Ferrari has a very fast car, but they seem to score only with one car or have more trouble in the race - but Mercedes is always there, so they are a very tough competitor."
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Alonso had closed up behind team-mate Lance Stroll in the closing stages in Barcelona on Sunday, but took to team radio to say he would not attack his colleague on track with comfortable points in the bag and no threats around them.
But with P6 and P7 representing a fall from where they have been competing for much of the season so far, the Spaniard hopes to see a turnaround in form when Formula 1 resumes again in Montreal.
"We didn't have the pace," Alonso said. "I think we were not fast enough with the soft, not quick enough on the on the hards at the end, so I don't think it was a thing on the strategy or anything, it was just a lack of pace today. Let's see why and try to get better for Canada."