• He said that the areas of improvement in the car were obvious, but would not reveal what they were.
• Red Bull's Max Verstappen set the pace from Ferrari's Charles Leclerc and Carlos Sainz and Aston Martin's Fernando Alonso.
Lewis Hamilton says he "felt the improvements" of his upgraded Mercedes after finishing sixth fastest in practice at the Monaco Grand Prix.
Red Bull's Max Verstappen set the pace from Ferrari's Charles Leclerc and Carlos Sainz and Aston Martin's Fernando Alonso.
Hamilton said: "The car was generally feeling good.
"It's a shame we weren't as close as I'd have hoped at the end but definitely felt the improvements."
He added: "It's not a place to test an upgrade."
Mercedes have introduced a design development to their car that features new front suspension, side pods and floor.
It is the first step in a series of planned improvements after accepting at the start of the season that they had made a mistake by pursuing a unique design direction that has left them uncompetitive for the last two seasons.
Hamilton admitted that Mercedes would not be able to find the pace to close the gap to the fastest cars in Monaco.
He said that the areas of improvement in the car were obvious, but would not reveal what they were.
He added: "It's very clear where the lack of performance is for me. We will talk about that in the debrief and try to figure out how we can [address that] within what we have.
"But hopefully this gives us a platform to improve on going forwards. In this session, we were close to half a second off. Maybe that could have been 0.3 seconds."
Hamilton ended the session 0.498secs off the pace and, when told by his engineer Peter Bonnington where he was losing out to the fastest cars, said over the radio: "Damn. The car won't go much quicker than that."
Mercedes had planned to introduce the upgrade at the Emilia Romagna Grand Prix last weekend, but that race was canceled as a result of flooding in Northern Italy.
Teams rarely choose to debut major redesigns at Monaco because the unique tight and twisty nature of the track means forming a clear impression of their effect is difficult.
Team principal Toto Wolff said: "At least our car is not worse than before. It hasn't done anything we didn't like."
Hamilton's team-mate George Russell was only 12th fastest. He had been about 0.2secs behind Hamilton before ending his soft-tyre running early. Others continued and set faster times as the track improved.
Russell said: "We're sort of almost forgetting about the upgrades. We'll worry about that next weekend [at the Spanish Grand Prix] and focus on trying to improve here."
What about the pace-setters?
The fastest time was disputed throughout the day by Verstappen, Sainz, Leclerc and Alonso.
Sainz had appeared to be the faster Ferrari driver until he crashed at the Swimming Pool section with about 20 minutes remaining in the session.
The Spaniard had just been demoted to third quickest by his team-mate Charles Leclerc when he clipped the inside barrier at the exit of the Swimming Pool section.
Coming to the end of his series of laps on the softest tyre, Sainz misjudged his entry to the second part of the Swimming Pool and hit the inside barrier with a glancing blow.
That broke his right front suspension and sent him into the barrier on the outside, in a virtual carbon copy of a crash Leclerc suffered in qualifying last year after setting pole position.
Verstappen ended the day 0.065secs quicker than Leclerc, who finally found some pace on his final lap on the soft tyres. Sainz was a further 0.042secs adrift.
Alonso was 0.22secs behind Verstappen but was forced to abort a lap that appeared to be set to threaten the Dutchman's time when he encountered traffic in the final sector.
Verstappen had been unhappy with the performance of his car in the first session, complaining of it bottoming over the bumps at high speed, but it was improved for the second.
The world champion said: "P1 was quite tricky, I was not happy with the ride of the car on kerbs and bumps. P2 was a lot better, but compared to the Ferrari especially we are still a bit lacking on the general ride of the cars - so, how it handles the kerbs, bumps and drops in camber. That is still something we need to work on for tomorrow.
"You can see they are very close and knowing once we go all to the limit in qualifying we need a bit more to stay ahead of them. Aston Martin are close as well."
Alonso said his car felt "good and easy to drive", adding: "Tomorrow everything is to play for but we should be one of the candidates [for pole]."
Hamilton was split from the top four by an impressive performance from Lando Norris in the McLaren, fifth fastest in a car that has struggled for much of the season.
"A decent day," Norris said. "It is nice to be back here, it is pretty awesome. As special as always. A few wall kisses today. Pace looked pretty good."
Verstappen's team-mate Sergio Perez, the Dutchman's only vaguely realistic title rival this year, was down in seventh place, saying: "It wasn't a great day in terms of pace in the car."
Valtteri Bottas, driving an upgraded Alfa Romeo, was eighth fastest, with the Alpines of Pierre Gasly and Esteban Ocon completing the top 10.
Alex Albon, who crashed his Williams at the end of the first session earlier in the day, missed much of the second as the team repaired his car.
He got out for a few laps at the end but ended up slowest of all.