Sainz cancelled his media commitments on Wednesday after feeling unwell.
He then finished seventh in Thursday evening's second free practice session, 0.628s behind pacesetter Fernando Alonso in the Aston Martin and three tenths behind Ferrari team-mate Charles Leclerc in fourth.
After the session, Sainz was seen having to compose himself in the cockpit of his SF-24 before finally getting out.
The Spaniard revealed he went through a "difficult 24 hours", having to take it easy in practice due to his condition.
"Obviously a very difficult day after feeling sick," Sainz explained. "The last 24 hours have been tough and difficult for me.
"Today was all about trying to get myself on track and learn as much as possible from the car without really pushing too much the limits as I was still a bit on the non-ideal side, but we managed to complete the programme without issues.
"Hopefully tomorrow I'm feeling better, even if probably I will not be 100%. But feeling better will allow me to be more on the limit and perform a good Saturday."
As a result of his illness, Sainz refused to read too much into either his short or long runs and felt Jeddah's high-speed blasts made the day even more demanding.
"Today it's very difficult to read into lap times," he added. "We will prepare everything and try to go for a good qualifying.
"The track is super high grip and high speed, so it also makes it tough from a physical point of view.
"It's a lot more demanding for the car but also for the driver.
"A tricky day, as I said, but we managed to get it done. Now we'll go and take some rest and come back tomorrow stronger."
Behind Alonso, Mercedes' George Russell set the second-fastest FP2 time.
Red Bull's world champion Max Verstappen, who remains the firm favourite to win this weekend's event, was third after likely using a conservative engine mode.
