The P Zero Yellow medium tyres were used throughout Q3 by Red Bull's Alex Albon and for the final Q3 run with both Mercedes drivers, to set their fastest times of the session and take pole with Lewis Hamilton.
Both Mercedes also set their best Q2 times on the P Zero Yellow medium tyres, meaning that this is the tyre they will start the race on. Ferrari driver Charles Leclerc additionally went through Q2 on the medium. All the rest of the top 10 on the grid will start on the P Zero Red soft.
Ambient temperatures were generally mild: more than 20 degrees with track temperatures just above 30 degrees and intermittent gusts of wind. Tomorrow's weather could be variable though, with a chance of rain.
The newly-asphalted surface in Portimao is very smooth and offers low grip, but should continue to evolve throughout the race.
Mario Isola: "There's an interesting variation in terms of strategy, with both Mercedes in first and second, as well as the Ferrari of Charles Leclerc in fourth, starting the race on the medium tyre: which also set pole. It's going to be fascinating to see which advantage the medium runners can take from this compound.
"With the gusting winds and blind crests, Portimao is far from an easy circuit. So we could have an unpredictable race tomorrow, where managing the tyres - the three hardest in our range, which was the right choice as today's action showed - and keeping them in the right temperature window will be essential in the relatively cool and low-grip conditions.
"We saw in qualifying that the track became a bit slower in the final part of Q3. In these decreasing track temperatures, along with gusting winds, we saw both Mercedes drivers and the Red Bull of Alex Albon opt for the Yellow medium tyres during the decisive qualifying runs. With a relatively unknown circuit and lack of long runs in a disrupted FP2 on Friday, it's tricky to clearly identify the performance gaps between the tyres as well as wear and degradation levels, which in turn makes it hard to predict the optimal race strategies."
Check out our Saturday gallery from Portimao, here.