Formula One world championship leader Lewis Hamilton will start from the middle of the pack at Sunday's Turkish Grand Prix after his Mercedes team decided to introduce a new engine component in Istanbul.

Hamilton, who leads Verstappen by just two points, will use a new internal combustion engine (ICE), his fourth of the season, Mercedes confirmed.

The news did not distract Hamilton from posting the best time of 1 minute 23.804 seconds around the 5.338-kilometre Istanbul Park circuit in Friday's second practice session, having also topped the first in the morning.

Drivers are allowed only three ICE units per year before penalties are applied.

But Hamilton will not start from the back of the grid like Red Bull title rival Max Verstappen in Russia two weeks ago as changing only a single component, rather than the whole engine, brings just a 10-place penalty.

"There is the balance and risk of a reliability issue, and obviously the thing that you definitely don't want to do is fail during the race and then have to take a penalty anyway," Mercedes trackside engineering director Andrew Shovlin told Sky Sports.

Hamilton starting in midfield is likely to hand the lead in the championship to Verstappen, who is currently only two points behind in the standings with seven races remaining.

Verstappen, aided by the rain-induced chaos which brought Hamilton victory in Sochi in the last race, fought back to claim second in Russia and minimise the damage to his title hopes.

Now Hamilton will attempt to do the same as he tries to win a record eighth world title as Verstappen chases his maiden championship.

Verstappen was second fastest in first practice but a modest fifth with 1:24.439 in the second session.

Charles Leclerc, of Ferrari, was instead closest to Hamilton at 0.166 seconds behind with Valtteri Bottas third in the other Mercedes and Sergio Perez fourth with the other Red Bull.

Leclerc's team-mate Carlos Sainz has also said he will have a full engine change and will start from the back of the grid.

Daniel Ricciardo, looking to continue his resurgence with McLaren, could only finish 14th in second practice, after his 12th place effort in the morning.