Billie Jean King, at the start of the festivities at Arthur Ashe Stadium Monday evening, noted to the crowd it was “Women’s Equality Day.”

King, one of Serena Williams’ most ardent supporters one year ago after “Serena versus The Umpire,” added, “But we have so much more to do until there is truly equality for all.’’

One year ago, battling for women’s rights became part of the narrative after Williams’ meltdown against umpire Carlos Ramos, who flagged the Compton, Calif., legend with three straight code violations.

On Monday’s opening night, a woman, Marija Cicak, was in the umpire’s chair. There were no blowups, no illegal coaching from Patrick Mouratoglou, no code violations and not much drama.

The USTA has barred Ramos from umpiring a Serena match during this U.S. Open. Asked about the ruling Monday night, Serena quipped,

“Yeah, I don’t know who that is.’’

Untested, Williams remained rather quiet Monday, letting her racket do the talking as she routed what is left of Maria Sharapova in a 59-minute 6-1, 6-1 first-round thumping.

If this match contained any history, it could serve as Sharapova’s last one at the Open, though sources believe the five-time Grand Slam champion will play another year as her Nike contract runs its course.

Unlike Andrew Luck, Sharapova, now out of the top 100 with this first-round loss, wants to battle on despite constant shoulder pain. She got hammered Monday, but lost with grace. Her opponent, for all her 23 Grand Slam victories, does not always do so.

“You can write me off,’’ Sharapova said. “There are many people that can write me off, especially after going down 6-1, 6-1 in the first round of the Open. As long as it’s not the person that’s inside of you, you’ll be OK. You can write me off, but that’s not me.’’

No, Williams and Sharapova weren’t equal Monday — except for the first 10 minutes. Sharapova held her serve to square the bout at 1-1 and let out a squeal of joy.

At least Sharapova evened Serena in the fashion war, wearing a charcoal tennis dress to Serena’s trademark one-piece black suit.

The sellout crowd of 23,771 cheered Sharapova, but gave Williams a more resounding ovation when she emerged on Ashe for the first time since September’s debacle.

Williams noticed.

“They were so amazing,” she said. “I could hear them walking down the hallway. It was such a good feeling.’’

In 24 minutes, Williams wrapped up the first set, painting the lines with perfect power and made just three unforced errors. She destroyed Sharapova’s second serve. She essentially has owned Sharapova for years, running her record to 20-2. If Williams owned her any more, she’d be CEO of Sugarpova – her candy company.

“I’ve had a lot of tough matches here, a lot of tough losses, but coming out tonight makes it all worthwhile,’’ Williams said in her on-court interview.

The toughest came against Naomi Osaka in last year’s final and sent the sports world into a dizzying debate on whether coaching should be allowed and whether umpires treat female players’ outbursts differently than they do men’s.

Serena was so confused after the mess, she sought a sports psychologist as she continues the quest to tie Margaret Court’s record of 24 Grand Slam titles.

Dr. Hillary Cauthen, board member of the Association for Applied Sports Psychology, has worked with professional tennis players and college players. She feels there’s plenty of evidence about a bias against women.

“She was expressing her emotions,” Cauthen told The Post when reviewing last year’s fiasco. “It’s a difficult place as a high-level athlete in high competition to manage emotions. As a female athlete, society looks at it differently and labels the emotions different. There’s truth to that. It’s a hard path. How can we change society’s perspective of how man and woman express their emotions?’’

Serena’s strokes and emotions were under control Monday with six matches to go before she can finally make history.