Lewis Hamilton has conceded the Mercedes W13 lacked "everywhere" during Friday practice for the French Grand Prix.
The seven-time F1 world champion finished second practice fifth, two-tenths down on team-mate George Russell after sitting out FP1.
But alarmingly for the Silver Arrows on a weekend the team was expected to mount its strongest challenge against Red Bull and Ferrari, Russell was seven-tenths slower than pace-setter Carlos Sainz with Hamilton a further two-tenths adrift.
"It wasn't the best of sessions," said Hamilton.
"I tried to get back up to speed as quick as possible.
"It is very hot so there are a lot of slow laps to get the temperatures of the tyres down but I think I did alright."
On where the Mercedes was lacking around the Paul Ricard lap, Hamilton conceded: "Just everywhere, every corner. We will have to dig deep into the data but it just feels like we are lacking downforce."
Hamilton "grateful" for de Vries input
Hamilton's absence in the first session of the weekend was part of Mercedes' obligation to run a rookie in two sessions across the season, as per F1 regulations.
Nyck de Vries - who performed similar duties in Spain for Williams - took to the cockpit and finished in the top 10, compiling valuable data for Hamilton's weekend.
"I did make some changes going into the session which is kind of crazy because I hadn't even driven the car," explained Hamilton when asked about the process of beginning his weekend in the second hour of running.
"Nyck did a great job this morning, kept the car in one piece and was great out there.
"I am grateful for the work he did this morning and we had a lot of work to do, a lot of ground to cover.
"The car is not spectacular here, don't know why but I am sure overnight we can make a bit of a step."