 Based on sales data from Amazon.com , Portal 2 was the best @-@ selling game in the United States in the first week of its release , but was overtaken by Mortal Kombat in its second week . According to NPD Group , Portal 2 was the second @-@ best selling game in the U.S. in April 2011 , at 637 @,@ 000 copies , and the fourth @-@ best selling in May . However , NPD does not include sales on Valve 's Steam platform . Portal 2 was the best selling game in the U.K. in the first week of its release , the first number @-@ one for a Valve game . It retained the top spot during its second week . 
