Simplify the following text for a Grade 7 native English-speaking person.
Original: The building ranked officially as the world 's tallest from 2004 until the opening of the Burj Khalifa in Dubai in 2010 .
Simplified: In 2004 , it replaced the Willis Tower ( which used to be called the Sears Tower ) as the tallest completed building in the world . However , in 2011 , it was surpassed by the Burj Khalifa in Dubai .