Valtteri Bottas headed Lewis Hamilton as Mercedes dominated first practice at the Spanish Grand Prix.Bottas was 0.039 seconds quicker than Hamilton, while Red Bull’s Max Verstappen in third was 0.939secs off.Ferrari’s Charles Leclerc and Sebastian Vettel were fourth and fifth, separated by just 0.011secs.Haas’ Romain Grosjean was a surprise sixth. Racing Point’s Sergio Perez was seventh on his return after missing two races because he caught coronavirus.The pace of the Haas cars, with Grosjean’s team-mate Kevin Magnussen in ninth place behind the second Red Bull of Alex Albon, was the big surprise of the day. They are usually towards the back of the field.Perez and team-mate Lance Stroll, who was 10th, were slightly out of position but they were the only drivers in the top 10 to use the medium tyres to set their fastest time rather than the quicker softs. Vettel showed improved form after struggling during the last two races at Silverstone. Ferrari discovered a crack in his chassis when stripping the car on Sunday night and have changed it for this weekend.The teams will have spent the session assessing the durability of tyres in the hot weather, after the struggles that cost Mercedes victory at Silverstone on Sunday.Pirelli has supplied the hardest of its three compounds for this weekend at the Circuit de Barcelona-Catalunya, while each tyre was one grade softer last time.Barcelona, like Silverstone, is tough on tyres because of its series of long-duration, medium- and high-speed corners, and the temperatures this weekend are forecast to be even higher than they were on 70th Anniversary GP race day.But at least in this first session there were no major indications of problems for any team, although Hamilton did say his front-left tyre was showing some signs of problems when he did a long run late in the session. McLaren’s Lando Norris was down in 13th place, two behind team-mate Carlos Sainz, while George Russell sat out the session for Williams to give development driver Roy Nissany some practice.The Israeli was slowest of all, 0.3secs slower than race driver Nicholas Latifi.