Are there single-celled organisms that have evolved from multi-cellular ones?
I would consider HeLa cells to be an example of a unicellular eukaryotic organism that evolved from humans. It can survive independently and replicate within cell culture plates, but cannot survive in the wild, however. 



HeLa cells are, like in your example, cancer cells, in this specific case human cervical cancer cells. They were propagated as an immortal cell line, and completely match the replication characteristics of eukaryotic cell lines when propagated in the laboratory.

It can be even considered the case that HeLa cells are actually a kind of unicellular eukaryotic model organism not unlike S. cerevisiae, since they are widely used in experiments involving human-like cells. 

As WYSIWYG has said in the comments, some biologists have even assigned the binomial name Helacyton gartleri to the cells (albeit with some controversy).