Is Mercedes edging closer to breaking point?

The drivers' and constructors' championships after four races make pleasant reading for Mercedes. Nico Rosberg has had a serene run, becoming only the fifth driver to win the first four races of the season, to lead team-mate Lewis Hamilton by 43 points.

Mercedes has accumulated an impressive 157 points, more than double that of its closest rival Ferrari and only two shy of its tally at this point last year. But look behind the results and you'll find evidence of a team that is starting to feel the strain.

As Rosberg said in Russia, the Mercedes F1 W07 remains the car to beat.

"It is special to have the car that I have for the third year in a row now," he said. "I don't think anyone has ever had that in the history of the sport either, to have such an awesome car for this period of time."

Maintaining that level of excellence will understandably start to take its toll. It's not feasible to maintain it forever.

Eventually, that level will drop. It's the way of the world. And after two utterly dominant seasons, in which Mercedes has won a staggering 36 out of 42 races, we've seen the first signs of that happening.

Rosberg won the Australian Grand Prix comfortably, but Mercedes chief Toto Wolff said after the race the German came close to retiring because of an overheating right-front brake caliper.

In China, Hamilton suffered a MGU-H problem in the first part of qualifying, which resulted in him starting at the back. Mercedes thought it had got to the bottom of the problem, issuing a detailed explanation of the process it goes through to diagnose and solve such an issue.

But in Russia, Hamilton suffered an identical failure in Q3. In the race, Hamilton's chase of Rosberg ended because of a water-pressure problem - Mercedes says he completed the last 16 laps with zero water pressure - while Rosberg encountered a problem on the MGU-K that meant he had to turn everything down.

If its rivals had been closer on this occasion, Mercedes would not have had the luxury of doing that.

Mercedes spent pre-season testing focusing on reliability rather than headline times. It amassed an impressive 1294 laps - 3743 miles - over eight days, 709 miles more than the next team (Toro Rosso).

And yet mistakes are creeping in. Wolff suggests it is because the threat from its rivals, particularly Ferrari, has intensified.

"Ferrari has made a huge step over the winter and this is why we are pushing boundaries," he said. "Sometimes when you reach the limits, you just need to find out. And this is where we are.

"Because we are pushing so hard, we had an issue on Nico's car where on the MGU-K, we saw some overtorque, which is quite a worrying signal because it might break."

Wolff also admits that Mercedes is struggling to adapt to the new radio ban, which limits what teams can tell its drivers.

"With the current regulations, you are not really allowed to tell the driver, so it was quite difficult to manage that," he says.

Those two factors, after a period of having things its own way, would be a challenge for any team - even one of the calibre of Mercedes, which has pushed the boundaries in terms of Formula 1 excellence in recent years.

Really, Mercedes is suffering from its own success. With the current regulations now in its third year, the gains Mercedes can make are smaller than those who are trying to close the gap. It means Mercedes must be more creative and that in turn and can lead to problems.

With the field contracting, the chances of Mercedes being able to plan a race that will essentially be between its two drivers are diminishing.

It has had to change its battleplan on track and adjust to the growing threat from behind. That throws more variables into the mix, and that in turn leads to more potential mistakes. Split-second decisions on strategy take on extra significance. The safety net has gone. Perhaps the team is having to turn up the engines more often than usual and that is bringing up problems it didn't need to encounter in 2014-15?

Another hint that Mercedes is feeling the pressure was its unusual decision to issue an open letter to the fans.

The team took a hit on social media over the Russian GP weekend as conspiracy theories swirled regarding Hamilton's string of mechanical problems.

Wolff batted away the claims in Russia, describing those suggesting such theories as "lunatics" and adding: "It's very difficult to take people seriously out there who are lying in their bed with a laptop on their chest and just sending out abusive messages."

But it's clear that by issuing this letter, Mercedes felt it needed to defend itself. Hamilton has felt the brunt of the unreliability this year, though there is plenty of evidence from the past where it tends to be one driver in a team for whatever reason that suffers this state of affairs. Mark Webber at Red Bull, Rubens Barrichello at Ferrari and Carlos Sainz Jr at Toro Rosso last season are just a few examples.

It was an incredibly detailed letter, full of emotion, which explained its situation.

Mercedes is not going to sabotage one of its drivers. It is ridiculous that anyone would suggest a professional organisation would do such a thing. But the fact it felt the need to defend itself in such a way shows to an extent the pressure it is under to keep delivering, primarily to its own board.

Its drivers have so far been the pillars of strength for the team this season. Rosberg is enjoying a supreme run of form, so that's understandable. But we've yet to see him prove that he is capable of soaking up the pressure from a sustained attack. That attack will come at some point this season.

Hamilton, meanwhile, has dealt admirably with the frustrations of 2016. He has yet to have a clean weekend in four attempts. It's a sign of Hamilton's maturity that he has kept calm and refused to get downbeat about the situation. But in Russia, despite publically keeping a "glass is half full" approach, there were the first signs that this run is starting to affect him.

The fact that it was an identical problem that afflicted him in Russia clearly hurt. He suggested he felt "helpless" adding: "The goal is moving further and further away in the distance. I'm doing everything I can do."

He remains positive for now, but the longer this run goes on, the greater the toll it will take.

The bare facts are that Mercedes tops both championships comfortably and left Russia with the maximum 43 points on offer despite having problems on both cars in the race.

But if Ferrari gets its act together, the chasing pack continues to get closer and Hamilton's patience runs out, there will come a time when Mercedes will be punished for its vulnerability.

And then we will see how severe the chinks in the team's armour could be.