Methods, systems, and software are provided for monitoring a cancer condition of a test subject. The method includes obtaining a liquid biopsy sample from the subject at a second time point, occurring after a first time point, containing cell-free DNA fragments. Low-pass whole genome methylation sequencing of the cell-free DNA fragments is performed to obtain nucleic acid sequences having a methylation pattern for a corresponding cell-free DNA fragment. The nucleic acid sequences are mapped to a location on a reference genome. Methylation metrics are determined based on the methylation patterns and mapped locations of the nucleic acid sequences. A circulating tumor fraction is estimated from the methylation metrics, and the estimate is compared to an estimate of the circulating tumor fraction for the test subject at the first time point.