While Hamilton topped the timesheets ahead of Red Bull's Daniel Ricciardo, Sebastian Vettel was only fourth fastest on what the Ferrari driver described as "a mixed day" for the Prancing Horse. "As I said, it's the same for all of us".
"It was very close but I am very happy", Hamilton said.
Bottas was a tenth of a second faster in the morning session than Hamilton's afternoon time at the 4.6-kilometer (2.8-mile) track outside Barcelona. Vettel won in Australia and Bahrain, while Ricciardo was first in China and Hamilton triumphed in Azerbaijan. I'm sure there have been a lot of people with nerves over the past few races, just not really understanding - constantly learning but feeling that we're not learning quick enough.
"So far we can be happy", Vettel said.
"This is the auto we should have had in Australia".
Vettel replied: "I think you should ask us what we need to overtake". But it's still a pretty long run, we will see.
Kimi Raikkonen: "It was a tricky qualifying, not the ideal one", was Kimi's comment.
During pre-season testing at the Circuit de Catalunya, a number of teams - most notably Mercedes - struggled with excessive overheating and blistering on the circuit's newly-laid track surface. "We still don't know what happened, but we'll figure it out".
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Fernando Alonso will start from eighth in McLaren's best qualifying display of the year.
The omens are also now positive for Hamilton and Bottas to go on and land a big result for their championship chances with the victor of Spanish GP only having come from outside the top two on three occasions since 1991.
Robert Kubica, Williams' reserve driver, drove in an official F1 session for the first time since nearly losing his right hand in a rally crash in 2011.
F1 recently announced changes to the wings and brake ducts to make it easier for cars to follow each other, which theoretically should improve racing.
It was also the first front-row lock up for Mercedes this season. Sergei Sirotkin was back in the vehicle for the afternoon run. "FP1 was especially promising, the vehicle felt good from the start".
Stroll almost hit the tire barriers after going off the track with about a half-hour left in the first session. "It's a bit like cruising to America and changing direction 100 times".
"These conditions make it very hard to define where the set-up needs to go".
Brendon Hartley never qualified at all after crashing his vehicle in the morning's free practice three, wrecking the rear of his Toro Rosso. Both Force Indias of Esteban Ocon and Sergio Perez, advanced into the second round, did not advance to the third.