The single album included "Playing with Fire", which reached number three and sold more than 2.5 million digital copies in South Korea.

On June 15, 2018, the group released their first Korean EP, Square Up, which debuted atop the Gaon Album Chart and was certified 2× platinum for selling 500,000 copies.

It was the group's first song to enter the US Billboard Hot 100 at number 55, making them the highest-charting female Korean act and the first to be certified gold in the country.

On October 19, the group collaborated with British singer Dua Lipa for the track "Kiss and Make Up", achieving commercial success worldwide.

The group subsequently released the second pre-release single “Ice Cream" featuring American singer Selena Gomez on August 28, which peaked at number 13 on the Billboard Hot 100, becoming the highest-charting as well as the longest-charting song by a female Korean act.

On October 2, The Album was released alongside title track “Lovesick Girls", which peaked at number two on both the Billboard Global 200 and the Gaon Digital Chart.