The longevity of Lewis Hamilton's career in Formula 1 will depend on his ability to sustain his motivation, says two-time world champion Mika Häkkinen.
The Mercedes driver achieved his fourth title in 2017, and while there are no doubts about his presence in F 1 in the next couple of years, with a massive new contract likely just around the corner, many believe he could call it a day after 2019.
Häkkinen however reckons Hamilton will stretch his stay as long as he has the energy and motivation to fight on.
"Lewis' achievement in winning four world championship titles is impressive, and you can see that he has kept the hunger alive, that is critical," Häkkinen wrote on his Unibet blog.
"When I won both my world championships in 1998 and 1999, I went into 2000 absolutely determined to win a third title, but it was not to be.
"After that year, when I felt my energy levels drop a little, it was not long before I retired."
Reflecting on the past season, Häkkinen admitted to have been mightily impressed by Max Verstappen, and the Finn also hopes Red Bull Racing will challenge more often for race wins in 2018.
"Max Verstappen was the standout driver of the season because he is so exciting to watch," he said.
"The two wins in Malaysia and Mexico showed that both he and the team have been getting stronger, and with Daniel Ricciardo winning in Baku, it means Ferrari and Mercedes could never relax."