At the zenith [ˈzinəθ] of her music career in the early 2000s, she released her best-selling album to date.

zenith
noun [ˈzinəθ]

Zenith refers to the strongest or most successful period of time for a person or thing.

When you reach the zenith, you're at the top, the pinnacle, the summit, the peak.

Zenith developed from an Arabic phrase meaning "the way over one's head," and then traveled through Old Spanish, Medieval Latin, and Middle French before arriving in English.

Figuratively, nadir simply means "the lowest point."

English