SEPANG, Malaysia - Spaniard Jorge Lorenzo shook off missing the final three races of 2011 due to injury and set the fastest time on the first day of pre-season MotoGP test runs at Sepang.MotoGP has moved to 1000cc bikes for the 2012 season from the 800cc power of earlier seasons.Yamaha dominated on Tuesday to open three days of tests here with three of its riders among the top four; 2011 champ Casey Stoner of Australia was sidelined by a strained back muscle as teams unveiled their new 1000cc bikes.MINUTE'S SILENCEThe test session was an emotional return for riders to the Sepang circuit for the first time since Honda rider Marco Simoncelli died here in a shocking crash in October's Malaysian MotoGP. The Honda team laid a wreath at Turn 11, where 24-year-old Simoncelli was killed, and stood silent for a minute.Lorenzo's 2011 season ended with a crash during the warm-up laps for the Australian GP but he set the fastest pace on Tuesday on his Yamaha in hot and humid conditions - a 2min01.657 lap. Countryman Dani Pedrosa (Honda) was next on 2min02.003.Yamaha's Carl Crutchlow posted 2min02.221 and his American team mate Ben Spies 2min02.234.Stoner failed to show whether he could maintain his form on the new bikes after pulling a back muscle while stretching just before morning testing.'PROPER SETTINGS'Pedrosa shrugged off the strong Yamaha performance, saying he was still getting the feel for his new Honda."I'm focused on myself. I was most of the time alone on the track and was not mixing with the other MotoGP riders," he said. "I hope to do better on Wednesday - we will start (to find) the proper settings for the bike."Test runs were delayed in the morning after rain left the track damp.Multiple World champion Valentino Rossi, who struggled through a difficult 2011, posted a fifth-fastest 2min02.392 on his Ducati Desmosedici GP12.Testing will end on Thursday but there will be another three-day session at Sepang from February 28.The MotoGP 2012 season will start on April 8 in Qatar.