"I need to be a better person on the court," said Medvedev, who was fined after each of his first three matches at Flushing Meadows.

The Russian fifth seed booked a berth in the last eight by defeating 118th-ranked German qualifier Dominik Koepfer 3-6, 6-3, 6-2, 7-6 (7/2) at Louis Armstrong Stadium.

Medvedev was fined $9,000 for an obscene gesture and unsportsmanlike conduct in a third-round victory Friday. After his middle-finger gesture was shown on video screens, fans booed him unmercifully.

"Talking about last match, I was an idiot, to be honest," Medvedev said Sunday. "I did some things that I'm not proud of and that I'm working on to be a better person on the court because I do think I'm a good person out of the court."

There was a bit more kindness Sunday from the crowd in the form of mostly hearty cheers for Koepfer, but when Medvedev had won and boos followed, Medvedev encouraged more and said jeers helped energize him.

"I was painful in my attitude," Medvedev told the crowd. "I was painful in my shoulder. I was wondering if I was going to play. And you guys gave me the energy.

"Guys continue to give me this energy. You are the best."

Medvedev says he is still working on being his best but learned some lessons after reading social media comments about himself after Friday's match.

"I do like to read comments about me, to learn what people think about me, where should I be better or what should I keep?" Medvedev said.

"To be honest with you, especially after the last match, I learned a lot to myself. As I say, I need to be a better person on the court than I was the last match."