MANAMA - Lewis Hamilton and Nico Rosberg hammered home Mercedes' early season dominance on Friday when they set the fastest times in opening practice for Sunday's Bahrain Grand Prix.Hamilton, the winner in Malaysia last weekend, clocked 1min 37.502sec to top the time charts with world championship leader Rosberg, who won in Australia, in second spot thanks to a time of 1min 37.733. Ferrari's Fernando Alonso, a three-time winner at the dusty Sakhir circuit, was third-fastest with Nico Hulkenberg taking fourth place in a Force India. Britain's Jenson Button, racing in his 250th grand prix this weekend, was fifth-fastest in the McLaren, ahead of Ferrari's Kimi Raikkonen.Meanwhile, four-time world champion Sebastian Vettel saw his early-season troubles continue with a 10th place in his Red Bull. The German was 1.887sec slower than Hamilton, while his Red Bull teammate Daniel Ricciardo was down in 14th place, 2.904sec off the leading Mercedes. Felipe Massa, twice a winner at Bahrain during his Ferrari days and the quickest driver on the track during winter testing last month, was the best-placed Williams man.His time of 1min 39.533sec put him in 11th place overall, still more than two seconds off the pace of Hamilton. Three reserve drivers made their first appearances of the season during the session, with Felipe Nasr taking the seat of Valtteri Bottas in the second Williams, finishing in 13th. Giedo van der Garde, in for Esteban Gutierrez in the Sauber, was 18th while Robin Frijns, sitting in for Kamui Kobayashi in the Caterham, was down in 21st. Marcus Ericsson, in the second Caterham, brought up the rear, more than five seconds off the leading time. The second practice session will get under way at 1500GMT and will take place under the floodlights to be used for Sunday's race. Alonso, for one, will be hoping for a smoother time in his Ferrari.Despite finishing third in the first session, 0.451sec behind 2008 world champion Hamilton, he had to overcome an embarrassing blunder by his pit crew who sent him out with a mix of soft and medium tyres. The Spaniard had to beat a hasty retreat to have the problem fixed.