When the U.S. Open begins on Monday at the Billie Jean King National Tennis Center, change will be in the air and on the ground.

Serena Williams enters the tournament as the women’s No. 1 seed and the No. 1 player in the world, but at age 34 this hasn’t been her strongest season — though by any standard a good one — and to think she would pound her way to the final again isn’t the safest of bets.

Novak Djokovic is the No. 1 seed and the undisputed No. 1 player, but the defending champion comes into Flushing Meadows after losing in the Olympics and pulling out of the Cincinnati tournament with a sore left wrist. Question is, will two weeks be long enough to recuperate and will that wrist hold up during the two-week grind?

These players are at the top of the rosters in the final major of the year, and will be playing under the new roof that appears to hover over Arthur Ashe Stadium, a roof that now should assure a Sunday afternoon finish. And nearly the whole of the south campus is brand-new this year, with a new Grandstand open for play and rebuilt field courts with additional seating, additional shade and additional amenities.

But while much of the Open’s staging is new, the cast of characters has changed.

Gone is Roger Federer, five-time Open champion, who after Wimbledon decided that he needed to take the rest of the season off to recuperate a bad knee.

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Gone is Maria Sharapova, banned by the International Tennis Association for two years for testing positive for a banned drug early this year.

Gone is Victoria Azarenka, a perennial contender who announced in July that she was pregnant and expecting a child at the end of the year. She had won the Indian Wells and Miami tournaments this year, then struggled with a knee injury.

Gone is the Open’s defending women’s champion, Flavia Pennetta, who after defeating Italian countrywoman Roberta Vinci in the final last year, announced her retirement.

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What isn’t gone is the expectation that Williams still has the ability, when healthy, to win a seventh Open title. Her blunderbuss serve, her atomic forehand and her fierce competitive drive are an unbeatable combination when her physique is sound.

But this season she has played in only seven events total, winning twice, at Rome and her 22nd Grand Slam title at Wimbledon. She lost in the final at the Australian to Angelique Kerber and the final at the French Open to Garbine Muguruza, both strong results and big disappointments. She was ousted early at Miami, a tournament that she has dominated, and at Indian Wells, a tournament she only recently returned to and one she should be a strong contender.

Her only competition since winning Wimbledon was at the Rio Olympics where she lost in the third round to Elina Svitolina. With the early exit at Rio, she got into the Cincinnati tournament with a wild card, then pulled out before the tournament began with a shoulder problem. She has had hip and knee woes throughout the year. Time and her ferocious playing style have taken a toll.

But if there is a change to come at the top of the women’s game, if Williams is to be dethroned at No. 1 by Kerber, the Open’s No. 2 seed and the only player who could overtake her, it won’t be without a fight.

“I haven’t played a lot, I haven’t practiced a lot, but I’m just now starting to feel a little better,” Williams said. “Hopefully just every day I will keep going higher.”