Serena Williams entered the U.S. Open with concern she wouldn’t win a match.

Now there’s thought Williams can win the tournament.

The Serena Show continues Friday night at what will surely be a rollicking Arthur Ashe Stadium when the unseeded 40-year-old legend plays her third-round match against Ajla Tomljanovic of Australia, trying to postpone retirement at least a couple of more days. If Williams wins, she will play on the Sunday afternoon card.

Williams turned back the clock in Wednesday’s night’s tennis party with a smashing performance over No. 2 seed Anett Kontaveit, playing scintillating tennis in the deciding third set.

Ashe Stadium has been as deafening as it’s ever been in its 25 years for her two matches. Friday may be no quieter with another celebrity-studded crowd expected. Tiger Woods, who inspired her latest comeback, was among her personal guests.

“I don’t think you can ever count Serena out,’’ ESPN’s Bethanie Mattek-Sands, who is on hiatus from the tour, told The Post. “She’s a gamer. She seems to be appreciating the journey and energy the crowd is giving her. And she looked really sharp.

“It’s going to be tough for the opponent to handle the crowd and the walkout and everything that is Serena. On top off all that, she’s moving well. [Wednesday] night her returns were keeping her in the match. She’s playing more defense with her movement and she’s tough to play like that.’’

Williams is listed as a -165 favorite over her Aussie opponent and 12-1 to win it all. She was 33-1 at the start of the tournament to win a record-tying 24th Grand Slam.

Her new coach, Rennae Stubbs, has been a strong influence.

“I just tried to get her comfortable,’’ Stubbs told The Post. “Now it’s just about tennis, and no one is better at winning tennis matches than Serena.’’

Her opponent is ranked 46th and knows she can make a name for herself by ending Williams’ career. Mattek-Sands is tour friends with the Aussie.

“She’s playing some great tennis,’’ Mattek-Sands said. “All the matches going forward, her opponents will know what the deal is and know the crowd is not for them. They have an idea what that walkout looks like. The first night, nobody knew how big it would be and it was intimidating. Ajla will be prepared for the crowd, knowing the only ones cheering for her are the family members in her box. And she’s going to be OK with that.’’

For all her bluster, Williams realizes at this stage she can lose to anybody. Her opponent said she wanted this moment so she could play Williams for the first time.

“She’s been playing pretty intense and I’ve been actually watching her matches a lot,’’ Williams said. “I saw her play last week. I think it’s going to be a good match. You asked the fact that she wanted to play me. I think it’s good. I have to be ready for every match.’’

After losing the second set Wednesday, Serena took a bathroom break and came out a new player, ripping service returns and darting around like she was 30 — not 40.

“I never thought I’d see this level of tennis from Serena again,’’ ESPN’s Pam Shriver said. “That third set was good enough to win the championship.’’

Maybe miracles can happen in Flushing.

“I cannot think that far,’’ Williams said. “I’m here, I’m having fun and I’m enjoying it. Honestly, I’ve had so many tough matches the last year I don’t know how long that I just feel like just being prepared for everyone that I play. I’ve been practicing really well, but it hasn’t been coming together in matches. But, you know, now it’s kind of coming together, I guess. I mean, I had to bring it together today. It worked out.’’