Ahead of today's opening session the big question is engine updates, not olny who will have them and will they make a difference.
While Adrian Newey has played down the significance of Renault's update, Lewis Hamilton has lamented the late decision to hold off on the Mercedes update, due, we are told, to a "quality control" issue.
All of which should play into the hands of Ferrari, which will be introducing its own upgrade as will Honda for Toro Rosso.
While the tyre compounds available are the same - hypersoft, ultrasoft and supersoft - as Monaco, Montreal is an entirely different circuit. Nonetheless, Mercedes, already hurting due to the lack of that engine upgrade, will be hoping that it is on top of the issues that plagued it in terms of the hypers in the Principality.
Just one reserve driver on duty today, Nicholas Latifi - so of McLaren shareholder Charles Latifi - replacing Sergio Perez at Force India.
Air temperature is currently 18 degrees C, while the track temperature is 30 degrees. Chance of rain is 0%.
Due to a lack of spares, both Haas drivers are on notice to keep it clean and stay away from the barriers, while Brendon Hartley is one driver in need of a decent weekend.
Alonso is in full test mode, the MCL33 fitted with a 'garden gate' sensors rake, as is Verstappen's Red Bull.
Indeed, the Dutchman is first out, followed by Raikkonen, Grosjean, Ericsson and Leclerc.
Gasly, among the first to venture out, warns that the track is "very dirty", as rivals confirm this by kicking up large clouds of dust.
An almost equal mixture of ultras and supers, though the Pink Panthers run the hypers.
While most pit at the end of their installation lap, Verstappen is one of several who stays out.
Five minutes into the session, all but the Mercedes pair have been out. Indeed, neither driver is even in their car, though they don't look inordinately concerned.
After that brief flurry of activity in terms of the install laps, there follows several minutes of nothing.
After ten minutes of inactivity, the sound of a Mercedes fills the air and seconds later Sirotkin heads out. He's on supers.
The Russian is subsequently joined by Hartley and Ocon, and eventually Verstappen.
Sirotkin posts the first time of the weekend, crossing the line at 1:23.556, subsequently improving to 20.427.
As Sirotkin raises the bar with an 18.798, Hartley posts 25.149, however, both are demoted when Ricciardo bangs in a 17.475.
As Ricciardo improves to 16.306, Gasly becomes the fourth driver to post a time, the Toro Rosso driver crossing the line at 19.059.
Twenty-three minutes into the session, Bottas is the first of the Mercedes drivers to head out.
Alonso, on the hypers, posts 15.240 to go quickest, as Stroll goes fourth.
Hartley complains of "very, very poor traction".
Grosjean goes third with a 16.798 as more and more drivers head out and the timesheet expands to 10 names.
Quickest in the final two sectors, Alonso improves to 15.006.
Grosjean enquires why there is "no grip going into the chicane".
Vandoorne posts 15.516 to make it a McLaren 1-2, as Hulkenberg slows to a crawl, the German claiming that the car "went out of gear".
Bottas goes third (15.640), ahead of Ricciardo and Hamilton (15.969). Sainz is sixth, ahead of Grosjean, Sirotkin, Gasly and Raikkonen.
With Hulkenberg stranded on track, first the VSC is deployed and then the session is red flagged.
Given a number of options to get the car going again, Hulkenberg insists that "I cannot get a gear". "It doesn't work, it doesn't react or anything," he wearily adds.
After just over thirty minutes, all but Verstappen, Ocon and Hulkenberg have posted times.
The session will re-start at 10:40 with permission given for the extra set of tyres to be used for a further 11 minutes.
Ericsson leads the field back on to the track as the session resumes. Hamilton hangs back for a practice start.
Bottas immediately goes quickest with a 14.509, as teammate Hamilton takes second with a 14.774.
An improvement from Ricciardo sees Alonso demoted to fourth, but the Spaniard responds with a 14.275 to go quickest again only to almost instantly lose out to Bottas who improves to 14.243.
Meanwhile the front brakes are being changed on both Haas cars.
A 14.064 sees Alonso go top again, as Hamilton leapfrogs Vandoorne with a 14.397. Both McLarens still on the hypers.
Raikkonen complains at how snappy his car is. The Finn is currently seventh, just ahead of teammate Vettel. However, the German subsequently posts a 15.033 to go sixth.
Again Vandoorne improves, the Belgian posting 14.334 to go third, only to be demoted when Hamilton bangs in a 13.967.
After missing the chicane, Latifi opts to abort his lap.
As the extra set of tyres are handed back, 51 minutes into the session, it's: Hamilton, Bottas, Alonso, Vandoorne, Raikkonen, Ricciardo, Vettel, Ocon, Sainz and Gasly.
Verstappen is yet to post a time while Hulkenberg is unlikely to.
The Dutch driver posts a 14.911 to go seventh, albeit on the hypers. He subsequently improves with a 14.158 to go fourth.
His brakes changed, Grosjean is back on track.
Quickest in all three sectors, Verstappen raises the bar with a 13.389. As he pits, the Red Bull driver passes the Renault crew pushing Hulkenberg's car back to the pit, albeit on the other side of the barriers alongside the St Lawrence.
"I touched the wall," confesses Stroll, the wall being the infamous 'Wall of Champions', the Canadian having given it a nudge with his right-rear which is punctured in the process. He is currently 17th, 2.957s off the pace.
Despite the fact that one of his tyres is almost hanging off, he makes it back to the pits having been warned not to let the tyre "unravel".
PBs in all three sectors see Ricciardo go third 13.640 as Verstappen complains of something hanging off his car. He pits, and it appears to be a piece of fairing on the left front suspension.
Still on the hypers, Ricciardo goes quickest in S1. A PB in S2 is followed by a poor final sector after a lock-up and as a result he fails to improve.
"Red Bull is getting quicker and quicker on hypersoft but what's interesting is that Hamilton's time for Mercedes (currently second) was set on ultrasoft," says Pirelli.
On the hypers, Sainz improves to seventh with a 14.116.
In 16th, Hartley is told he's done a good lap but needs to improve in T10. Nonetheless, the kiwi is currently quickest in the speed-trap, on 328.5 km/h, ahead of the Mercedes pair, Ericsson, Gasly, Leclerc and Latifi.
Still only 13 laps on the board for Vettel who is currently ninth, 1.731s off the pace. At which point he heads out again.
On the hypers, Vettel improves to seventh with a 14.106.
Another improvement for Vettel who goes fourth with a 13.574.
Double yellows are waved after Ericsson spins at T8. Elsewhere, Hartley is off at T1 after losing downforce behind a Force India.
A late spin for Sirotkin also as the Russian goes into the barrier backwards at T6, though there appears to be no real damage.
The session ends but there's still time for another spin from Ericsson, this time at T8. Moments later, Sainz spins at T9.
Verstappen is quickest, ahead of Hamilton, Ricciardo, Vettel, Bottas, Raikkonen, Alonso, Sainz, Vandoorne and Gasly.
Ocon is eleventh, ahead of Grosjean, Ericsson, Leclerc, Magnussen, Hartley, Sirotkin, Stroll, Latifi and Hulkenberg.
Biggest talking point after that, other than the fact the track remains very dirty, and the Honda, in particular, appears to be a step forward, is the fact that for the first time this year, old 'butt-head' lead the way.
Check out our Friday gallery from Montreal, here.