The victory over the 80th ranked Kovinic, just her second this year, will be a confidence boost for Williams but the path to a record equalling 24th Grand Slam now gets treacherous.
Waiting in the wings is Estonian second seed Anett Kontaveit, who breezed past Jaqueline Cristian 6-3 6-0.
Williams indicated her intention to retire in a Vogue article in early August, saying she was “evolving away from tennis” but never confirming the U.S. Open as her final event.
For tennis fans, however, the message was clear, the U.S. Open would be where she would take her final bow.
Could there be an encore?
The former-world number one has played coy even refusing to rule out next year’s Australia Open.
But in a strange post-match ceremony celebrating her career, which was not yet finished, Williams gave the clearest sign yet that the U.S. Open will indeed be her last tournament and expanded on that later in her press conference.
“It’s extremely difficult still because I absolutely love being out there,” said Williams. “The more tournaments I play, I feel like the more I can belong out there.
“But it’s time for me, you know, to evolve to the next thing.”
Pressed if the U.S. Open was definitely her last event Williams again stepped back from the brink.
“I’ve been pretty vague about it, right,” smiled Williams. “I’m going to stay vague because you never know.”
A montage of Williams career played before she appeared on court that left the door open a crack for a return some day, “if you ever decide to return Queen your throne will be waiting” ended the video.
A defiant Williams, however, made it clear she was not giving up that throne just yet.
There is always excitement on opening night at Flushing Meadows but on Monday the teeming stadium crackled with a special energy from the moment Williams appeared on court decked out in a shimmering black robe and specially designed diamond encrusted shoes.
The outfit may have sparkled more than the 40-year-old American’s play but it did not matter to a jam-packed Arthur Ashe Stadium as Williams’ fighting edge remained razor sharp even if her serve and ground strokes were not.
“It’s so important to give your all,” Williams told the adoring crowd. “I’ve been down and out so many times, in the public eye.
“I just want people to be inspired by my story.
“I’m from Compton, California… and I made it.”
Certainly there was no more fitting place to bring the curtain down on one of tennis’s most phenomenal careers than in a city that has been in her corner from the very beginning, fuelling runs to six U.S. Open crowns.
The magnitude of the moment was not lost on the 23,000 fans, including, former U.S. president Bill Clinton, designer Vera Wang, director Spike Lee and others who packed into Arthur Ashe not expecting to see greatness but to celebrate it.
Having been world number one for 319 weeks, Williams arrived in New York ranked below 600 and unseeded.
Even with Williams far from her best the odds were stacked against the 27-year-old from Montenegro.
Playing in her 21st U.S. Open, Williams has never lost in the first round and her victory over Kovinic was her 106th at Flushing Meadows.
While Kovinic has been enjoying the best Grand Slam season of her career reaching the third round of both the Australian and French Open, she had not won a single match since Roland Garros.
As play got underway it was Williams showing signs of nerves, piling up the double faults as Kovinic got in front 3-2.
But Williams, as she has done so many times, lifted her game when she needed to, sweeping the next four games to snatch the first set.
Now in charge, Williams would not let it slip away in the second set, breaking to go up 3-2, and with match point and the crowd on its feet, she danced in delight as Kovinic’s return hit the net.