Sergio Perez continued his strong form to outpace Ferrari's Charles Leclerc and his Red Bull team-mate Max Verstappen in the opening practice session at the Azerbaijan GP.
Perez, buoyed by victory at the Monaco GP two weeks ago and the subsequent announcement of a two-year contract extension with Red Bull, was 0.1s faster than Leclerc, with Verstappen a further 0.2s back.
The other Ferrari of Carlos Sainz trailed Verstappen in fourth, while Mercedes endured a challenging session in Baku as Lewis Hamilton could only manage sixth behind Fernando Alonso's Alpine, with team-mate George Russell eighth behind the AlphaTauri of Yuki Tsunoda.
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In addition to being behind his team-mate, there was further concern for Verstappen as his violently flexing rear wing appeared to attract the attention of rival teams, and looks set to be investigated by the FIA.
The issue appeared to occur when Verstappen opened his rear wing in the DRS zone, but Perez did not face the same issue. The Dutchman previously suffered DRS problems during the Spanish Grand Prix, and has previously expressed his frustration at the reliability issues the team have faced this season.
Speaking to Sky Sports F1 after the session, Red Bull team principal admitted that changes may need to be made to Verstappen's setup to satisfy officials.
"Obviously we've seen it, we've got the data on it, we've got the comparison to Sergio's wing so I think there's some modifications that we need to do just to make sure we eradicate that for this afternoon," Horner said.
"Weight is always a critical factor and you're chasing that, obviously, all over the car.
"There was a difference between the cars so we just need to look at the set-up of it and the rigidity of it and hopefully we can get on top of it for this afternoon."
With F1's all-new 2022 cars facing a street circuit for a second successive race after last month's trip to Monaco, ride quality proved an issue as most teams struggled with severe bouncing on the straights.
Perhaps the most significant difference between the circuits are the long straights in Baku, which encourage teams to run lower downforce than in Monte Carlo.
Several drivers expressed frustration via team radio throughout the session, but few more so than Verstappen, who arrives in Baku with a nine-point lead over Charles Leclerc at the top of the world championship, with Perez six points further back.
Despite his frustration, it was Verstappen who topped the timesheet for much of the session, before Perez's 1:45.476 propelled him into first with 20 minutes remaining, with Leclerc moving into second soon after.
Mercedes came into the weekend with hope they could produce a performance more akin to their improved display in Barcelona last month than their struggles in Monaco that followed the Spanish GP.
However, problems were quickly apparent for the reigning constructors' champions, with significant changes to their front suspension being carried out less than half way through the session.
There was some minor encouragement for the Silver Arrows in the closing stages of the session, with Hamilton climbing to fifth before being pushed down a place by Alonso, but the Brit's time was still almost 1.2s off Perez's leading effort.
Having come into the weekend under pressure following a series of heavy crashes, Mick Schumacher's session lasted just seven minutes before a pressure leak forced him to park his Haas at Turn 12.
Schumacher's technical issue was one of two brief pauses to the session, with the Williams of Nicholas Latifi also drawing a virtual safety car after a sudden loss of power saw his session come to an end after completing just six laps.
It was also a disappointing start to the weekend for McLaren, but particularly for Daniel Ricciardo, who's future with the team has been a topic of much speculation following criticism from chief executive Zak Brown ahead of the Monaco GP.
Lando Norris could only manage 11th, but was still more than a second faster than Ricciardo, who was trailed by only Latifi and Schumacher.