Nico Rosberg signalled his intent in final practice for the Spanish GP as the German set the pace ahead of Sebastian Vettel.
The weekend's final practice began with a Ferrari 1-2 as Kimi Raikkonen and Vettel set the pace early on as the field waited for the Mercedes drivers to emerge.
20 minutes into the 60-minute session, Raikkonen was quickest on a 1:27.330 while Vettel was 0.510s behind his team-mate as they lapped on the hard Pirelli tyres.
Daniel Ricciardo was the first driver to swap to the mediums as the Aussie, looking to make up track time after Friday's engine woes, hit the front. A 1:27.048 for the Red Bull driver saw him edge Raikkonen by 0.282s.
However, rather ominously for Ricciardo, almost immediately his time was beaten by the Mercedes pairing as Rosberg went quickest on a 1:26.777 while Hamilton posted a 1:26.965 – and they did so on the hard tyres.
But proving that even World Champions are fallable, a mistake from Hamilton saw him spin at the exit of Turn 3. He ran a bit wide, put wheels on the dirt and lost control flat-spotting his Pirellis.
In the final 15 minutes, with the track temperature up from 19'C to 32'C, the field changed over from the hard to the medium tyres for qualifying simulations.
As expected Mercedes stayed on top with Rosberg setting a benchmark time of 1:26.021. However, the reigning Champs were not able to achieve the 1-2 as a lock up late in his lap cost Hamilton.
He finished 0.201s down with Vettel separating the team-mates.
Valtteri Bottas brought his Williams home in fourth place while Raikkonen's efforts were undone by a "wrong setting." Informed of the issue by his pit wall, he sarcastically responding with an "excellent, excellent".
Ricciardo was sixth ahead of Felipe Massa and Max Verstappen
There were more issues for Red Bull Racing but this time they befell Daniil Kvyat. A water leak spotted prior to FP3 meant he ventured out late in the session and managed just seven laps. He still finished ninth while Carlos Sainz completed the top ten.
1 Nico Rosberg Mercedes 1:26.021 19 laps
2 Sebastian Vettel Ferrari 1:26.177 0.156 14 laps
3 Lewis Hamilton Mercedes 1:26.222 0.201 10 laps
4 Valtteri Bottas Williams 1:26.682 0.661 19 laps
5 Kimi Raikkonen Ferrari 1:26.944 0.923 14 laps
6 Daniel Ricciardo Red Bull 1:27.048 1.027 19 laps
7 Felipe Massa Williams 1:27.109 1.088 19 laps
8 Max Verstappen Toro Rosso 1:27.132 1.111 18 laps
9 Daniil Kvyat Red Bull 1:27.313 1.292 8 laps
10 Carlos Sainz Toro Rosso 1:27.809 1.788 18 laps
11 Jenson Button McLaren 1:27.938 1.917 18 laps
12 Nico Hulkenberg Force India 1:28.082 2.061 13 laps
13 Felipe Nasr Sauber 1:28.096 2.075 23 laps
14 Fernando Alonso McLaren 1:28.304 2.283 17 laps
15 Romain Grosjean Lotus 1:28.578 2.557 18 laps
16 Pastor Maldonado Lotus 1:28.618 2.597 19 laps
17 Marcus Ericsson Sauber 1:28.788 2.767 20 laps
18 Sergio Perez Force India 1:28.996 2.975 14 laps
19 Will Stevens Manor 1:31.125 5.104 16 laps
20 Roberto Merhi Manor 1:31.749 5.728 17 laps