Lewis Hamilton said his preparations for the Malaysian Grand Prix have been "heavily compromised" by reliability problems with his Mercedes.
The world champion, chasing an eighth win in nine races on Sunday, missed Friday's first session and some of the second but still set the fastest time.
"Whenever you lose a session - and pretty much half of the second - it definitely doesn't help," he said.
"But the team did a great job to rebuild the car and that was crucial."
Despite his problems - caused by a failure in the engine's inlet system and issues with telemetry - Hamilton was 0.428 seconds quicker than team-mate Nico Rosberg.
He admitted that was "good, for sure".
But he added that he would head into final practice before qualifying on Saturday having not made any changes to his set-up, and said the balance of the car had been "quite a bit off from where I need it".
|Andrew Benson's Malaysian GP view|
|"Malaysia vies with nearby Singapore as the toughest grand prix on the calendar.|
|"Both take place in crippling tropical heat and humidity. Singapore is longer and bumpier, but takes place in the slightly cooler conditions of night and does not have the demanding high-speed corners of Sepang.|
|"Carved out of former jungle, Sepang is the first and arguably best of the new generation of Formula 1 tracks. Designed by in-house architect Hermann Tilke, it has pretty much got it all - long straights for overtaking, demanding high-speed corners - especially the sweeps of Turns Five and Six - and the tricky 12-13-14 section.|
|"It would be a track that tested drivers to the limits of their abilities even without the punishing conditions."|
Hamilton goes into Sunday's second race of the season (at 08:00 BST) seven points clear of likely title rival Rosberg, after the Mercedes drivers finished one-two in Australia two weeks ago.
The Briton said Ferrari looked like being Mercedes's closest rivals after Kimi Raikkonen split him and Rosberg with the second fastest time and produced what appeared to be a strong race-simulation run.
"The Ferraris look great," Hamilton said.
"It's surprising to see how good their times are. We'll see whether that continues through the weekend.
"My lap wasn't spectacular. There are some improvements I can make with the balance and the settings but they were just all brought from the last race. I'm sure we will improve it a little bit.
"I got a fairly decent lap but we haven't dialled the car in, changed the differential settings or changed the set-up at all. So there is definitely some work to do.
"My tyres seemed to be lasting quite well and hopefully it will be good this weekend."