Red Bull's motorsport consultant Helmut Marko says he's still mystified by Daniil Kvyat's steep decline in form.
The young Russian made his mark by storming to the GP3 Series championship in 2013. That earned him a promotion to the Toro Rosso F1 team in 2014, and then to Red Bull in 2015 alongside Daniel Ricciardo.
But Kvyat was demoted back to Toro Rosso in 2016 in a straight swap with the latest rising star, Max Verstappen. Although the team retained him for 2017, he was eventually dropped for Singapore and Malaysia. His contract was finally terminated after the US Grand Prix.
"[He showed his speed] in his first year with Toro Rosso and in his first year with Red Bull Racing," Marko told Motorsport.com. "After that his performance drastically went down.
"He had brake and tyre issues that Ricciardo didn't have. In short, something happened to him mentally. He lost his speed and his ease. We don't know [what the reason was].
"We tried many things, but his speed just wouldn't come back," Marko continued. "Unfortunately we have no idea what happened.
"There were also too many accidents at the start of a race.
"He didn't react too well to it either," Marko added. "He retreated into his shell and didn't want anybody to tell him what to do."
"It's sad, because I remember GP3 in Monza and Spa where he completely outperformed the competition," he said.
"At Toro Rosso there was also one race where he started from behind and made his way through the field," he recalled. "He was attacking Raikkonen when his brakes failed. He had some sensational races."
Marko even agreed with the suggestion that at his height, Kvyat was a faster driver than either of Toro Rosso's current drivers Pierre Gasly and Brendon Hartley.
Toro Rosso principal Franz Tost had previously suggested that Kvyat's form collapsed because the driver put himself under too much pressure. However, Tost said that he still hoped that Kvyat would one day bounce back.
"I hope for him that he gets another chance," he said. "I think he deserves to be in Formula 1.
"Maybe a short break to get organised again, and probably we will see Daniil back at his usual performance level with another team."
Kvyat's only remaining hope of being back for the 2018 season opener lies with a seat at Williams. However it seems that compatriot Sergey Sirotkin is all-but assured of the race seat, pending an announcement in January.
Gallery: The beautiful wives and girlfriends of F1 drivers
Keep up to date with all the F1 news via Facebook and Twitter
