Lewis Hamilton was not able to push the car as he would have liked
Lewis Hamilton managed 49 laps in the morning session and finished third on the timing sheets behind Williams' Felipe Massa and Red Bull's Daniel Ricciardo.
But the Brit was frustrated by the feel of the car and also struggled with the tyres in the heat.
"It wasn't feeling great today, but that's just with set-up and it's quite hot today as well so tyres degrading and things like that," Hamilton told Sky Sports.
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"And we had some damage to the floor, but I didn't have any offs or anything like that, which is a good thing.
"I did see puffs of smoke from other people going off, but I managed to keep it on.
"Just a bit of an awkward balance today and I think the temperatures are a bit higher today, so it's harder on the tyres."
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Lewis Hamilton is favourite to win a fourth world title in 2017
Mercedes enjoyed a near-perfect first week of testing with Hamilton and Valtteri Bottas both peppering the top of the leaderboard in Barcelona.
And while they struggled on the fifth morning of testing, Hamilton was keen for the team to get the car back into tip-top condition.
"They're working on that to make sure Valtteri has a clean afternoon," Hamilton added.
"I'm not particularly excited at the moment, just because it's the first day of week two and I didn't get a lot of running this morning, so I'm looking to the next couple of days."
Mercedes will hope to get as much running as they can even though they did manage to complete full race simulations for each driver during week one.
They will after today have just three days of testing left ahead of the Australian Grand Prix curtain-raiser in Melbourne on March 26.