George Russell and Mercedes started the Mexican Grand Prix on top, going fastest in a fraught opening practice session that featured two red flags.
Russell topped FP1 on a 1m17.998s, just over eight tenths of a second adrift of Charles Leclerc's pole position time from last year.
Sainz was Russell's nearest competition, ending up just over three tenths behind Russell. Yuki Tsunoda completed the top three for RB, seven tenths off Russell, with Max Verstappen fourth.
Verstappen lost the final minutes of the session as his Red Bull team assessed his RB20 for an issue.
“Something wrong with the engine mate, I have no power," Verstappen told his race engineer Gianpiero Lambiase shortly before pitting.
"There's something wrong with the engine mate.."