World Champion Lewis Hamilton led a Mercedes 1-2 during the opening practice session for the Brazilian Grand Prix at Interlagos.
Finnish team mate Valtteri Bottas was second in both and ended up just 0.048 slower.
"Unfortunately the tyres don't last very long so you only get one or two laps on the short runs". McLaren has struggled at Brazil in its troubled time with Honda and the Interlagos circuit has provided two of the more iconic images of the partnership - Alonso relaxing in a deckchair in 2015 and then doubling up as a trackside cameraman previous year, both after engine failures.
"We'll see what we get tomorrow. We're in a decent position but I think it could be quite close". Max Verstappen, who claimed his second win of the campaign in Mexico last time out, was fifth quickest ahead of Ferrari's Kimi Raikkonen.
"I think they (Renault) are really struggling to understand what are the root cause of these issues", said Horner.
"This is not an opportunity for us to take our eye off the ball", added the Briton, who was planning on visiting the Inca citadel of Machu Picchu in Peru with friends in between the Latin American races.
"I think overall it's pretty straightforward: Mercedes seems to be the quickest and then it's pretty close between Red Bull and us", he said. Will team orders come into effect at Mercedes?
"Obviously, both cars have got bits that they (the engineers) are having a look at for future events", said Horner.
Lewis Hamilton may have won the Drivers' Championship, but he isn't out of the spotlight just yet.
But Hamilton was blisteringly fast when he switched to the "soft" tyre - more than 0.5secs a lap on average quicker than Bottas, who was comfortably quicker than Ricciardo as the three were all out on track at the same time.
British driver George Russell, the 19-year-old GP3 champion, made his grand prix weekend debut with Force India and was an impressive 12th fastest at an unfamiliar circuit and in a vehicle he had not driven before.