Verstappen bolted on the soft tyres in the final part of the session to leapfrog long-time session leader Fernando Alonso, setting a 1m27.428s. The championship leader - who can sew up the title in Saturday's sprint race - then saw off an initial attempt from Charles Leclerc as the Monegasque suffered a snap of steering in his opening sector.
Sainz drew close with a 1m27.762s, but this was only good enough for second overall as Leclerc popped up into third on a second run with the soft tyres.
High winds and sandy residue on a new track surface contributed to a slippery circuit, causing Sainz to quip "my dad [rally champion Carlos Sainz Sr] would be quick in these conditions".
This ensured that the early times were well off the pace of F1's last visit to the Losail circuit in 2021, where Verstappen topped that FP1 session with a 1m23.723s. His ultimate pole time that year was a 1m20.827s, leaving him almost seven seconds short of that benchmark at the end of practice.
Fernando Alonso had topped the session ahead of the final half-hour, and his subsequent 1m27.919s was good enough to deliver fourth overall despite the obvious grip advantage of the softs around the refurbished Qatar circuit.
This was a tenth clear of Sergio Perez's time, as the Mexican could not get particularly close to his team-mate over the hour-long duration.
Yuki Tsunoda's late lap was enough to thrust the Japanese into sixth place overall, as AlphaTauri continued to show solid form after the introduction of its latest upgrade package in Singapore.
Source: Autosport