Lewis Hamilton has praised his Mercedes team's "incredible" performance in the first Formula 1 pre-season test.
The Briton and German team-mate Nico Rosberg produced record mileage in their new car in Barcelona, leaving rivals stunned at its consistency.
"I've never seen anything like it. Just the strength of this car, it just keeps going," said world champion Hamilton.
"After winning two championships, it's so easy to lose focus - but everyone has done an even better job."
The Mercedes ran so reliably that the team had to start using their two drivers on each day, rather than alternating them, to avoid tiring them out.
Hamilton, 31, added: "It's just been an incredible weekend for testing for us. Before we came here they were talking about doing 800km (497 miles) a day and I thought that was crazy because I've never done that in the past.
"There are no breakdowns and a lot of reliability."
Mercedes also tried a new nose design on Thursday, the final day of this week's test, with a number of unusual aerodynamic features.
Hamilton said: "The front wing is an improvement, which is good. It's always about small increments and that's what it's brought."
Rosberg, 30, and Hamilton ended the final day seventh and eighth fastest - Mercedes have concentrated on durability running and have not done any performances runs on the softer tyres.
"The car looks fantastic," added Hamilton. "Obviously we don't know the pace of the car just yet but what we can say is that it's gone the distance and that's half the battle."
Who was fastest?
Kimi Raikkonen's Ferrari set the pace, with a lap of one minute 23.477 seconds on the new ultra-soft tyre, 0.8secs quicker than Red Bull's Daniil Kvyat on the same rubber.
Force India development driver Alfonso Celis was third fastest and Kevin Magnussen an impressive fourth for Renault on the soft tyre, 1.8secs off the Ferrari.
Renault had a strong day, Magnussen managing more than 150 laps despite a couple of on-track stoppages, and the team said they were pleased with the car's performance so far.
At the other end of the times, Indonesian novice Rio Haryanto has had a difficult introduction to F1 with Manor.
He spun on his first day in the car on Wednesday, without hitting anything, and did so again on Thursday, this time crashing into the barriers at Turn Four. He was unhurt.
More McLaren-Honda woe
McLaren suffered a significant blow to end the first of only two pre-season tests this year as Fernando Alonso managed only three laps before the team found a water leak in the car.
They worked to try to get the two-time champion out again but ran out of time and Alonso ended the day without a lap time.
The problems rekindled unpleasant memories of 2015, when both pre-season testing and the season itself were plagued by unreliability and poor performance.
Honda is to introduce a new specification engine, intended to be a significant improvement, for next week's final test.
1 Kimi Raikkonen (Fin) Ferrari 1:23.477
2 Daniil Kvyat (Rus) Red Bull 1:24.293
3 Alfonso Celis (Spa) Force India 1:24.840
4 Kevin Magnussen (Den) Renault 1:25.263
5 Max Verstappen (Ned) Toro Rosso 1:24.393
6 Felipe Nasr (Brz) Sauber* 1:26.053
7 Nico Rosberg (Ger) Mercedes 1:26.187
8 Lewis Hamilton (GB) Mercedes 1:26.295
9 Felipe Massa (Brz) Williams 1:26.483
10 Esteban Gutierrez (Mex) Haas 1:27.802
11 Rio Haryanto (Idn) Manor 1:28.266
12 Fernando Alonso (Spa) McLaren no time
* = 2015 car
Raikkonen, Kvyat and Celis set fastest times on super-soft tyre; Magnussen, Celis, Nasr and Haryanto on the soft; all others on medium.