Sergio Perez continued Red Bull's strong start to the Azerbaijan GP weekend by setting the pace in Practice Two, with Mercedes surprisingly outside the top 10 and well off the pace in a session that also saw Charles Leclerc crash his Ferrari again.
Fresh off hugely contrasting weekends in Monaco as Red Bull took the championship lead off a below-par Mercedes, the title challengers were expecting the title holders to bounce back around the high-speed Baku streets - but Friday's running has suggested anything but.
Perez led an impressive Red Bull one-two in second practice to back up Max Verstappen's fastest time in the opening session - with Lewis Hamilton and Valtteri Bottas all the way down in 11th and 16th.
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Hamilton was one second off Perez's 1:42.115 while Bottas finished two seconds back.
A clearly agitated Hamilton, who also struggled in Monaco, told his Mercedes team on radio: "I don't really know where all the time is if I'm honest."
Asked about Mercedes' difficult day afterwards, Hamilton said: "It was clean, I got all the laps that we needed, there were not really any mistakes...but just slow. There was no more time in it."
Bottas added: "I think there's something fundamentally wrong and we need to figure out what."
In Mercedes' absence at the top of the timesheets, Ferrari provided the biggest threat to Red Bull on Friday and Carlos Sainz and Charles Leclerc were close to championship leaders in third and fourth in P2.
But Leclerc's session was soured by a crash into the barriers at Turn 15 - which broke his front wing off while also ruining a lap that may well have been enough for the fastest time of the day.
Leclerc also crashed his Ferrari two weeks ago in Monaco, and two years ago at the same Baku circuit.
Pierre Gasly was also very competitive in his AlphaTauri in fourth, while Fernando Alonso seems to be hitting form as the Spanish two-time champion finished Friday in sixth.
Antonio Giovinazzi was a surprise name in the top-seven in his Alfa Romeo, with Lando Norris the faster McLaren in eighth. Daniel Ricciardo, who was quicker than Norris in P1, was 13th in second practice.
Nicholas Latifi caused the only red flag of the session after grinding to a halt in his Williams due to an engine failure.
Why were Mercedes so slow? And can Red Bull capitalise?
After dominating races in Portugal and Spain, Monaco appeared to be an outlier for Mercedes as they struggled around the twisty and unique layout with their W12 car, with Red Bull and Ferrari benefitting.
But Mercedes have so far failed to move back towards the front in Azerbaijan. Worse yet, they have gone backwards - and, confusingly, at a track where they have won three of the last four races.
What has happened?
Hamilton, who said the car felt better in a P1 session he finished seventh in and over the long runs, was certainly perplexed.
"I feel like I'm driving well," stated the world champion. "But we are definitely quite a chunk down and everyone will be scratching their heads and looking into the data trying to figure out how we can improve."
Bottas, meanwhile, suspected the team are lacking grip.
"It was very tricky for us today, clearly lacking pace," he said. "It feels like it's overall grip... the balance is not that far off.
"OK the car is a bit unpredictable, but it feels like we're lacking grip and sliding around. I think it's going to be a long night tonight.
"We seem quite a bit off, it's more challenging than we anticipated and we need to find out exactly why."
It would be an even bigger surprise than their Friday woes if Mercedes were not to improve for Saturday's crucial qualifying day - although the size of the deficit has certainly opened the door for Red Bull and Ferrari to capitalise.
Red Bull were beaming after Friday as their RB16B looked fast on both the qualifying and race simulations, and even better news for the team is that Perez had similar pace to championship leader Verstappen all day.
"It was probably Perez's best session of the year," admitted team boss Christian Horner to Sky F1.
Ferrari, meanwhile, expected to be "back to reality" in Baku this weekend after such a strong Monaco, but instead once again look to be contenders for the front of the grid - particularly fast through the twisty second sector.
McLaren have ground to make up in the battle to lead the midfield, with AlphaTauri and Alpine also quick.
The action continues on Saturday, with final practice at 10am before qualifying at 1pm - all live only on Sky Sports F1.