Honing his craft at Toro Rosso
Sainz made his F1 debut at the 2015 Australian Grand Prix, qualifying in seventh place and going on to successfully finish in the points in ninth while his new team mate and fellow debutante Max Verstappen suffered an engine issue and failed to last the distance. However the balance of power between the two slowly shifted over the course of the season, and by the end Verstappen came out on top in 12th place in the championship with 49 points compared to Sainz' 18 points in 15th.
Along the way, Verstappen finished in the top four in Hungary and US which made him the apple of Dr Marko's eyes, and in turn made it impossible for Red Bull not to call him up to the senior team when Daniil Kvyat's brief tenure there came to an abrupt end after a series of mishaps for the Russian in his home Grand Prix in 2016. As the older driver in the team, Sainz must surely have felt rather miffed to have been overlooked for promotion - the first sign that his F1 hopes might not be met by staying put in the Red Bull camp.
It could easily have put him on the wrong side of Dr Marko, whose iron fist is rarely softened by a velvet glove
Losing momentum can be deadly to a F1 driver's career: staying too long in one place without achieving some sort of dramatic breakthrough consigns them to a the sidelines from which it's very hard to get noticed and earn a second chance to make a winning impression. Having seen the fate of previous Toro Rosso high fliers suddenly cast low - the likes of Jean-Eric Vergne, Vitantonio Liuzzi, Sebastian Bourdais, Jaime Alguersuari and Sebastian Buemi - Sainz was acutely aware that the clock was ticking and time was running out for him to find a successful way forward as Sebastian Vettel, Daniel Ricciardo and Max verstappen had previously managed.
His murmurings of discontent could easily have put him on the wrong side of Dr Marko, whose iron fist is rarely softened by a velvet glove. Just when it looked as though Sainz was set for a damaging schism with the team he was able to get a mid-season transfer to Renault as part of a complex deal over power units, with the ultimate aim of freeing McLaren from its nightmare Honda collaboration. Sainz was finally on his way - but where would it lead?