Rihanna's "Diamonds," the lead single from "Unapologetic," notched the top spot on the Hot 100, tying the Barbadian singer with Madonna and The Supremes for the fourth-most chart-topping singles in Billboard history.

Only Michael Jackson, Mariah Carey and The Beatles have had more hit singles.

The 24-year-old singer has just wrapped up a seven-day mini-tour across seven world cities to promote "Unapologetic," her seventh studio album, which was released on Monday and shot to the top of the iTunes album charts in 43 countries despite garnering mixed reviews.

"Unapologetic" scored 65 out of 100 on review aggregator site Metacritic.com, with some critics unsettled by the singer's harder sound and close-to-home lyrics. Others have embraced it.

The New York Times' Jon Caramanica called Rihanna's latest "one of her best albums yet," adding that the singer's presence made her "without peer when it comes to her image, which is steely and unforgiving and bulletproof."

But BBC Music's Natalie Shaw found the album a difficult listen, calling it "the sound of a human dragged headfirst into a breakdown," and criticizing the singer for using "pop music to inadvertently condone abuse."

The track that Shaw refers to and has everyone talking is Rihanna's collaboration with ex-boyfriend Chris Brown, who was charged with assaulting her three years ago.

In a 90's pop-inspired track called "Nobody's Business," the singers further fuel speculation that they have rekindled their romance, singing lyrics such as "your love is perfection" and "you'll always be mine."

Critics have found more evidence of Rihanna drawing on her tumultuous relationship with Brown on "Love Without Tragedy/Mother Mary," which follows "Nobody's Business" on the album. In it, Rihanna sings "felt like love struck me in the night," and makes references to "one drive, injured us fatally," possibly alluding to the night Brown assaulted her.

Rolling Stone magazine's Jon Dolan gave "Unapologetic" three and a half out of five stars, noting that Brown was "like a co-writer throughout."

Rihanna made sure to capitalize on the success of her biggest hit to date, last year's Calvin Harris electro-dance production "We Found Love." On "Unapologetic," the singer brings French DJ David Guetta on "Right Now," a thumping dance track.

The singer also reteamed with rapper Eminem on "Numb," following their 2010 hit song "Love The Way You Lie." Unlike the emotional content of domestic abuse on their previous collaboration, the Barbadian singer and Detroit rapper lightened the tone for "Numb," singing about love in the club scene.

The songstress also softened up for two intimate piano-led ballads, "What Now" and "Stay," in which she recruited newcomer singer Mikkey Ekko for an stripped-down performance, using the opportunity to showcase her voice.

(Reporting by Piya Sinha-Roy; Editing by Elaine Lies and Lisa Shumaker)

• A Permanent Economic Emergency