Alexander Zverev learned a valuable lesson successfully defending his Washington, DC title: age is irrelevant.

The 21-year-old Zverev swept weary 19-year-old Aussie Alex de Minaur to claim his ninth career title at the Citi Open.

More: Zverev Defends DC Title

World No. 3 Zverev was the oldest of the four semifinalists in Washington, DC and stands more than a decade younger than world No. 1 Rafael Nadal, who is 32, and soon-to-be 37-year-old Roger Federer.

"At the end of the day we all play against each other,” Zverev said. “It doesn't matter how old you are, how tall you are, when you started playing tennis, it matters what your ranking is and how well you play.

“We don't play under 21 or under 18 events anymore. “We all are a big group, a 36-year-old Federer can still play me, I can play an even younger player than me."

Still, Zverev, who reached his first Grand Slam quarterfinal at Roland Garros in June, doesn't put himself on the same level as the iconic champions.

"Roger and Rafa are still the best out there and still competing for Grand Slam titles," Zverev said. "I haven't won a Grand Slam title. Saying I'm on their level wouldn't be fair to them."

Photo credit: Kathryn Riley/BNP Paribas Open