The Williams Formula 1 team will run the latest specification Mercedes engine during this weekend's Malaysian Grand Prix.
Mercedes introduced an updated engine for its works team at the Belgian Grand Prix, with the improvement believed to have focused on performance.
As Italy was back-to-back with Belgium, there was not enough time to get units to its customers Williams and Force India for that race.
Both teams decided against fitting the engines at the following grand prix in Singapore to save mileage as the benefit would have been lower given the nature of the circuit.
As a result, Williams will give Lance Stroll and Felipe Massa the fourth and final engines of their respective allocations for Sepang with Force India, as expected, doing the same for Sergio Perez and Esteban Ocon.
All four units are understood to be exactly the same specification as the ones run by the Mercedes works team.
In addition to the new engine, Williams has brought some small aerodynamic tweaks to Malaysia, with the combination of improvements coming at a time when the team is locked in an increasingly title battle for fifth in the constructors' championship.
Williams leads Toro Rosso by only seven points, with seventh-placed Renault only 10 further back and Haas just five more adrift.
"Anything can happen, I certainly hope [we can hold onto fifth]," said Stroll.
"Looking at the upcoming tracks, it might be tricky at some, but I'm positive we can hold it.
"Even on tracks we were expected not to score points, we've pulled a couple out of the bag, like in Singapore.
"Even if the car may not be in its best shape at every circuit, it doesn't not mean we have to write off the weekend."
The battle for fifth
Toro Rosso, 52