As he switched from Toro Rosso to start his new works Renault adventure early, the Spaniard outperformed his experienced teammate Nico Hulkenberg in the US.
“His first weekend with Renault was almost perfect. Fast and aggressive,” said the French sports daily L’Equipe.
Also impressed was former F1 driver Juan Pablo Montoya, who was in the Austin paddock as a guest.
“I was really impressed with what he did,” the Colombian told Spain’s AS newspaper.
“It is not easy to get into a new car, not make mistakes and be competitive,” Montoya added.
“Clearly Sainz is now among the best.”
Renault’s bosses were also happy with the 23-year-old’s performance.
“Carlos was extremely motivated and very prepared, and he was aggressive but also very intelligent,” team boss Cyril Abiteboul said.
However, Montoya thinks Sainz’s performance might actually have contributed to the demise of his former teammate Daniil Kvyat.
Kvyat has again been dropped from Toro Rosso’s lineup after Austin, even though the Russian performed well last weekend alongside new team recruit Brendon Hartley.
Montoya said: “I think it’s sometimes not fair for the drivers in the Red Bull programme.
“Carlos beat Kvyat and it was said that maybe the Russian is not good enough. Perhaps what happened is that Sainz did a great job, not just at Toro Rosso but now at Renault, a works team.
“For Carlos it’s very good,” Montoya added, “because at Red Bull there are drivers that are sometimes wasted. But Sainz has managed to continue in F1 and with a great team.”