The chair umpire who penalized Serena Williams in the U.S. Open final is back at work.

Carlos Ramos is working the best-of-five Davis Cup semifinal series between Croatia and the United States in Zadar, Croatia.

"I'm just focusing on this tie and working again. That's all I can say," Ramos said Friday.

Ramos officiated the second singles match between Marin Cilic and Frances Tiafoe, which was completed without incident. Ramos calmed the raucous crowd on several occasions and came down from his chair to check a few ball marks in the clay but otherwise had no impact on the match, which Cilic won in straight sets to give Croatia a 2-0 lead.

Tiafoe, who was making his Davis Cup debut, didn't have any complaints.

"I didn't know Ramos was sitting in the chair. I really wasn't paying attention," Tiafoe said. "I was more worried about the person across the net than the official."

U.S. captain Jim Courier added: "We thought the officiating was excellent all day long."

Ramos gave Williams three code violations in her straight-sets loss to Naomi Osaka last weekend, and the American great argued she wasn't being treated the same as some male players.

USTA President and CEO Katrina Adams, who defended Williams, was overheard apologizing to Ramos on the sidelines of Thursday's draw ceremony.

Ramos wouldn't go into details over his discussion with Adams, who initiated the conversation.

"You know I cannot talk about that," Ramos said.

Williams made an appearance Friday, too, in Las Vegas for a business convention. She talked about her fashion business and said not a word Friday about gender equality in sports or the argument with Ramos.

Williams spent 25 minutes talking onstage with Sarah Robb O'Hagan, chief executive of Flywheel Sports, at the National Retail Federation trade show.

Also Friday, a tennis official who has worked 38 consecutive U.S. Opens said some officials are considering boycotting a Williams match.

"We're looking for an apology from Serena to the official or officials in general," said Bob Christianson of San Diego. "And if we don't get that, there might be a potential boycott of her next match.

"We've never had to go to this extent. But officials are scared. They're worried what happened to Carlos could happen to them. You do your job and you're condemned for it."