In an interview with ESPN past year, Alonso gave the following reasons for walking away from F1.
"It is true we had a couple of failures during testing, so it is something that is a weak point of the vehicle", he said.
That auto will be driven by Sainz and Lando Norris on Tuesday before Norris has the vehicle to himself on Wednesday.
"When I said previous year "bye-bye" it was because I felt this chapter was already complete".
"I think the vehicle has no weaknesses apart from the basic ones, compared to the top teams", he said.
Alonso announced his decision to leave F1 at the end of 2018 after saying he wanted to pursue other goals, such as victory at the Indy 500 with a McLaren-entered auto. On face value, it holds little interest for a two-time world champion, but Alonso said he hopes to help McLaren understand its vehicle.
"I guess they thought it is a good opportunity for me to try the vehicle but not take the driving time of the other drivers", Alonso added.
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Ronaldo joined Juventus from Real Madrid last summer, bringing an end to his nine-year stay in La Liga. Messi added: "I loved the one I scored vs Madrid in the Champions League semi-finals".
"When I said goodbye, it was because I achieved what I wanted to in formula one".
"Honestly, I'm not missing too much at the moment as I've been very busy".
"Maybe if I was relaxing at home maybe it was different but I have been testing and preparing different things so there is not much time to miss anything".
"On Pirelli it's more about testing the tyres and maybe [I will] have a little bit of an idea of what the auto is doing this year and being involved a little bit from last year in the development in the vehicle is useful for the team so I hope it's a productive test".
"I don't close doors 100 percent to anything in the future".
Alonso is now competing in the World Endurance Championship and, after winning at Le Mans a year ago, will bid to complete motorsport's Triple Crown by topping the podium at the Indy 500 in May.