Lagergren finished six strokes under par after an eagle on the 12th hole. Englishman Aaron Rai shares the lead with Lagergren.
Star player Ludvig Åberg is tied for third place, one stroke behind the leading duo. The 26-year-old finished the day with three straight birdies.
For superstar Jon Rahm, the day was considerably tougher. The Spaniard finished six over par and is tied for 136th, which means he is second to last in the competition.
The PGA Championship brings together several of the biggest names in golf and is part of the DP World Tour, formerly known as the European Tour.
The winner receives just over 15 million kronor in prize money. Last year, it was Alex Norén who won the lucrative competition.
