Has Mercedes regained the advantage already? And could Ferrari spring a surprise?

After Mercedes defied the odds to beat a faster Red Bull to a narrow victory at the first in Bahrain, the world champions finished the first day of practice at Imola looking like the favourites.

But was it a case of returning to business as usual or will Red Bull hit back in qualifying?

A drivershaft failure prevented Max Verstappen from setting a competitive lap time during the second practice session at Imola on Friday afternoon, paving the way for Mercedes to finish the opening day of the Emilia Romagna Grand Prix in first and second.

Valtteri Bottas beat Lewis Hamilton to the fastest time -- as he did in qualifying at this circuit last year -- but neither driver appeared to push the limit on their hot laps.

Sergio Perez was a second off the pace in fifth place for Red Bull, with F1's two Italian teams, Ferrari and Alpha Tauri, filling the 0.860s void between the Mexican and the two Mercedes.

With Verstappen's Red Bull breaking down before he had a chance to show its potential, it all looked a bit like last year.

But don't be fooled, the 2021 Emilia Romagna Grand Prix still has plenty of potential to surprise.

A home advantage?

Bottas' 1:15.551 on the medium compound tyre sat proudly at the top of the timesheets at the end of second practice, but it wasn't the fastest lap on Friday.

That honour went to Charles Leclerc, who set a 1:15.367 -- 0.184s faster than Bottas -- only to have his time deleted for exceeding track limits on the exit of Turn 9, known as Piratello.

GPS data tells us Leclerc was able to carry 5km/h more speed through Piratello than Hamilton on their respective fastest laps using soft tyres, but it's hard to tell whether that speed advantage came as a result of Leclerc running wide or Ferrari running a more powerful engine mode.

Split times reveal that the middle sector of the lap, in which Piratello is located, was the only one of the three sectors where Leclerc had an advantage over the Mercedes drivers, so it's fair to assume Mercedes has plenty in reserve and was able to make up a significant amount of performance in other corners around the track.

What's more, neither Mercedes saw the expected 0.5s lap time gain from switching from mediums to softs, firming up suspicions that it had its engine turned down for its fastest laps.

Mercedes' sandbagging should not cast a shadow over Ferrari's progress, but just helps to place Leclerc's deleted lap time in the correct context.

Over a nine-lap run on medium tyres, Leclerc's average lap time was a 1:19.232 before he ended the session in the wall at the second Rivazza.

Apologising to his team on the radio, Leclerc said: "Sorry, guys. I was pushing really hard."

His performance up to that point on his long run was eclipsing that of the two Mercedes drivers, which were averaging times in the 1:19.4s using the same tyre compound but was almost certainly down to a lighter fuel load.

Yet the fact a red car is in the right ballpark again acts as proof of the progress that is being made just 90kms down the road from Imola in Maranello.

"The car feels a lot better in these conditions, it feels great this weekend, even if we compare it to the last year, if you compare it, it feels much better," Leclerc said. "The balance is better, the grip is very good and I have quite a bit of confidence in the car, so I'm looking forward to tomorrow."

But if Ferrari is targeting a best-of-the-rest finish behind Mercedes and Red Bull at its home race this weekend, it will face tough competition from the only team with a closer factory to the Imola circuit, AlphaTauri.

Pierre Gasly finished the session third fastest, just 0.078s off Bottas, and did so without abusing track limits.

GPS data again backs up the theory that Mercedes was running in a low power mode as he found 0.55s of his lap time against Bottas on the straights, but the AlphaTauri appears to be a quick car at this circuit.

Gasly also backed up his one-lap pace with a competitive long-run.

He spent the majority of his high-fuel running on the soft tyre, which looks like a weaker race tyre compared to the medium Leclerc was using, but when he also switched to the medium at the end of the session, he immediately set two competitive lap times quicker than Leclerc's average on the same compound.

Ironically, any hope of gaining more data to compare with the Ferrari was stopped short when Leclerc's accident caused a red flag and ended the session five minutes early.

Does Mercedes have the edge over Red Bull at Imola?

Verstappen's absence from second practice makes it incredibly difficult to draw any conclusions about the battle between Mercedes and Red Bull.

The Dutch driver finished 0.058s off Bottas' fastest time in the morning practice session, indicating the pace was in the car, but enters the race weekend on the back foot after missing the most important track time in the afternoon.

Teammate Perez struggled to get the car hooked up over a single lap, leaving him fifth fastest, but his long-run pace looked more promising and suggested Red Bull will be close to Mercedes if it stays dry on Sunday.

The track layout at Imola and the significantly cooler conditions compared to the opening round in Bahrain seem to have played to Mercedes' strengths, and, if anything, appear to have given the world champions a slight advantage.

"[The car balance] is so much better than Bahrain," Mercedes team principal Toto Wolff said. "The car seemed together this morning and in the afternoon.

"Probably the temperatures helped a little bit because that's where we were struggling particularly in Bahrain. For a Friday, I am very happy to see that.

"It was one of those [driver] debriefs that are really nice because they said the balance feels in the right place with the understeer, exit oversteer but generally they feel very good with the car."

Hamilton added: "I think the team has definitely worked really hard to try and understand what happened in the last race where we were weak and see if we can try and tweak the car a little bit better with set-up.

"But I also think the track has also come a little bit more in our direction."

Perez, who missed out on over 20 minutes of the first practice session after tangling with Esteban Ocon in an incident that was not caught on camera, also noted a difference to the last race, but was confident Red Bull would find more performance ahead of qualifying.

"Very different, very different issues to Bahrain," Perez said. "In a way just getting better with it. Every lap that I do I really learn a few tricks here and there.

"Overall, more performance to be gained tomorrow... which I think we know where to find it and hopefully we can be much higher up for qualifying because the race pace seems to be strong."

Once again, the battle at the front looks finely poised, with Mercedes holding a slight edge thanks to a smoother two practice sessions for its drivers on Friday.

However, any form guide derived from the opening two practice sessions could be completely skewed if Sunday's forecast of rain holds true.

For the second race weekend in a row in 2021, it's simply too close to call.