It was another session that was dominated by changeable weather, Alonso on track when conditions were at their best.
The Spaniard logged a 1:15.810s to head George Russell and team-mate Lance Stroll at the chequered flag.
However, like Free Practice 1, a drying circuit and bursts of rain meant competitive times were more down to luck and, to a degree, session programme than necessarily a reflection of actual pace.
The session started dry, though rain was closing in, prompting a rush on track as the pit lane opened.
Max Verstappen headed the pack out as Russell reported worsening rain back to the Mercedes pit wall.
Carlos Sainz was fourth in line and, halfway around the lap, reported it was too wet for the medium tyres he was on.
Charles Leclerc delayed his entry into the session, took to the track on a set of intermediate tyres.
That caught the eye of the officials who noted him for a ‘tyre usage’ issue – the suggestion being he’d used the grooved rubber before the track was declared wet.
Conditions quickly stabilised, affording an opportunity to bank laps on slick tyres.
Pierre Gasly took the opportunity and soon went fastest as he became one of just two drivers to have recorded a timed lap – the other being a tardy 1:34 from Lewis Hamilton.
Tyre temperature was the key differentiator. With it, drivers had decent grip and could generate decent lap times. Without it, they struggled for grip in the slippery conditions.
That left Fernando Alonso fastest with a 1:15.810s, a time set just prior to the halfway marker.
The Spaniard had been on track for some time and built temperature in his tyres – pace coming towards the end of a longer stint more than a single lap, as is traditionally the case.
As such, there were busy periods with all 20 cars on track at times before the Il de Notre Dame fell silent around the half hour mark.
At a time when teams would typically be completing single-lap runs, they instead remained in the garage as the weather turned.
Eventually, drivers were tempted back out on track, but on the intermediate tyres.
It was clearly slippery, Leclerc spinning at the Turn 10 hairpin where he performed a neat donut to get going once more.
In the final minutes, conditions began to improve once more but not quickly enough to afford teams an opportunity to switch back onto slick tyres.
However, it highlighted how rapidly the circuit dried – in just a handful of minutes it had gone from far too wet for slicks to showing a clear dry line in points.
Nobody did brave slicks and when the chequered flag fell Alonso was fastest from Russell and Stroll.
Verstappen was only 18th best, as he lost around half the session when he suffered an ERS issue in the Red Bull Racing.
That forced him into the pits and out of the session as the team worked to solve the issue that left the car smoking on track.
Given the conditions, the actual loss of running was not significant, while the conditions made the timesheets a reflection of who was on track for a longer period at the right time than who has genuine pace.
A final hour of practice remains on Saturday, with conditions expected to remain much the same heading into qualifying.
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