A surprise new album by Kendrick Lamar opened Sunday at number one on the US chart in the latest triumph for the critically acclaimed rapper.

The album -- the fourth by the 28-year-old, entitled "untitled unmastered." -- sold 178,000 copies or the equivalent in streaming in the United States in the week through Thursday, Nielsen Music said.

Lamar released "untitled unmastered." without previous notice on March 4 in the wake of the Grammys, where he was the music industry gala's biggest winner with five awards.

He characterized the album as a series of rough works from the studio, although the songs take on heavy themes including an opening track that mixes images of the biblical apocalypse and contemporary social ills.

Lamar also went to number one in 2015 with "To Pimp a Butterfly," a reflective look at the state of black America that experimented with jazz elements, which are also apparent on "untitled unmastered."

The latest album hit number one despite the lack of a physical release. A CD came out on Friday, seven days after the digital version and too late to be considered for the first-week chart.

Discrepancies between digital and physical release dates, which once enraged record stores, have become increasingly commonplace as the popularity of streaming transforms the music industry.

Rap superstar Kanye West has said he has no plans to release a physical version of his latest work, "The Life of Pablo," and vowed to stream three new albums per year.