Top seeds Serena Williams, Novak Djokovic aren't at their best

— -- NEW YORK -- Serena Williams has a sore right shoulder. Novak Djokovic's problem lies on the opposite side, in his left wrist. The main question hovering over the US Open's top singles seeds may not be which of them will win the event and bag yet another historic title, but which of them will last longer.

"I'm doing everything in my power, with obviously the medical team, to make sure that I'm as close to 100 percent as possible during the course of this tournament," defending men's champion Djokovic said Friday at the National Tennis Center, ominously adding, "... at least for the beginning of it."

Williams, who has played just three singles matches since she won Wimbledon, echoed Djokovic's sentiments when she said, "I have not played a lot, I haven't practiced a lot, but I'm just now starting to feel a little better. Hopefully just every day I will keep going higher."

Each of the Grand Slam icons is coming off a severe disappointment at the Rio Olympic Games, their early losses attributed partly to their injuries. They're trying to balance the demands of practice with those of rehabilitation. Which is the priority?

"Somehow [I'm] managing to do both," Williams said, enigmatically. "Definitely am trying to practice more because I don't want to go into an event without having practiced. ... [And having] a lot of physical therapy."

Williams said the last time she felt that her serve was a fully functioning weapon was at Wimbledon, where the women's final was contested July 9. She may have relied overly on it while earning her record-tying 22nd Grand Slam title in London. But it clearly was not that familiar, lethal tool during her three matches in Rio, where she and sister Venus also saw their unbeaten Olympic doubles streak broken in the first round at 15 matches (and three gold medals).

The big Cincinnati combined event, the most significant of the US Open tune-up tournaments, followed immediately on the heels of the Olympic tennis event. Williams arrived on site and practiced once before withdrawing because of her shoulder. She said Friday that she's physically as fit as she's ever been but is concerned about her lack of recent playing time.

"I think usually I prefer to play more [matches] coming into the final Grand Slam of the year," she said. "But there's nothing we can do about it. You just have to make the best of every single opportunity. That's all I can do now."

Djokovic is being equally philosophical in the wake of his tribulations. His glorious 2016 suddenly blew up in his face during the third round of Wimbledon and later at Rio. On Friday he confirmed that "private" problems unrelated to his game or fitness contributed to his poor performance at Wimbledon. It was the first time he lost in the first week at a Grand Slam event since 2009.

The loss and its attendant issues launched the 29-year old Serb on an an emotional voyage that he said helped him "to evolve as a human being." He emerged from it refreshed. He blasted his way through the field at the Canada Masters without losing a set. "It took a while to get centered," he said, "But I started to feel great again."

At Rio, though, the wrist injury flared up out of nowhere just a few days before the start of singles. Ironically, Djokovic's first-round opponent was Argentina's Juan Martin del Potro, whose own multiple wrist injuries have ruined what might otherwise have been a hall of fame career. Del Potro, on the comeback trail once again, eliminated Djokovic. Then Djokovic and partner Nenad Zimonjic were knocked out in the second round of the doubles.

The losses were devastating for a player who considers himself an ambassador for his native Serbia. Djokovic still has just one Olympic medal, a 2008 singles bronze he earned in Beijing. A US Open title would certainly help him overcome his Olympic disappointment, but Djokovic, like Williams, is in a race against time.

"I'm just hoping that Monday when the tournament starts I'll be able to get as close to the maximum of executing my backhand shot as possible," Djokovic said, referring to the shot most affected by his injury. "Sometimes what you need as an athlete is time, but because the US Open is around the corner, I don't have too much time."

If he needs cheering up, he might try lobbing in a call to Williams. She left Flushing Meadows in a depressed daze last year after she was upset in the semifinals by heavy underdog Roberta Vinci. The loss killed Williams' quest to earn a historic calendar-year Grand Slam just two matches shy of the goal.

That match has vanished in Williams' rear-view mirror. She said Friday that she's very happy to be back at the National Tennis Center, where she's had so many "wonderful memories and moments." She has likewise relegated her recent Olympic failures to her trash file.

"At the end of the day I knew I gave the best effort I could [in Rio], and it just wasn't enough," she said. "I just practiced two days before playing my match. It's not ideal, but it was all I could do. So, you know, I'm starting to feel better now, and that's really positive."