(CNN) -- He's the greatest player in the modern tennis era, perhaps even of all time, but is Roger Federer's stellar career fading to twilight?
The 17-time grand slam champion is "in a very vulnerable state" ahead of his opening match at the U.S. Open on Monday, according to top coach Nick Bollettieri.
Federer regained the world No. 1 ranking with last year's record-equaling seventh Wimbledon title, but since then he has struggled with both his game and his fitness, and is seeded just seventh for the season's closing grand slam in New York.
"Roger Federer is great for the game. He's fantastic on court and fantastic off court. He's well respected, and he respects all the opponents that he plays," Bollettieri told CNN.
"Roger is in a very vulnerable state. He's in a vulnerable position because he's moved down to No. 7 now. Remember Pete Sampras went through a tough period. Fortunately for Pete he won a big one before he left the tour.
"My Andre Agassi went from No. 1 to No. 142 in the world, and he left on a pretty good note. What we don't want to remember is Roger Federer leaving on a low note.
"He's been fantastic, he moves beautifully, he does everything with ease. This is a big tournament for Roger Federer."
Being seeded so low, the 32-year-old faces a possible quarterfinal clash with his old rival Rafael Nadal, who recovered from serious knee problems to retain his French Open title in June and has now bounced back from his shock first-round loss at Wimbledon with Masters victories in Montreal and Cincinnati.
"He is playing unbelievable, he has brought a new dimension to his game," Bollettieri said of Spain's world No. 2. "He can move back eight to 10 feet or he comes forward and hits the ball very early.
"He's improved his serve and he's a lefty -- he's very dangerous. He's full of confidence, so watch out!"
The power of players such as Nadal is making it so much harder for the classically elegant Federer, says Bollettieri.
The Swiss has this year experimented with using a bigger racquet such as his rivals employ, but has switched back again.
"When you get a Nadal hitting those heavy crosscourt lefties, and then serving out wide, it is difficult," said the 82-year-old, who has coached 10 world No. 1 players across the men's and women's game.
"However, I believe that the U.S. Open and the Australian Open (in January) is going to tell the story."
Federer is coached by Paul Annacone, a former student of Bollettieri who also worked with Sampras for several years.
"I believe right now Roger cannot win just standing on the baseline. I believe he has to come in," Bollettieri said.
Bollettieri is concerned that an underwhelming end to his career could undo the Federer legacy.
"It would be a shame if people forgot who he was," he said. "Look at what he brought to the game. He brought class. He lived a beautiful private life.
"He's quiet. He's always respectful of the sport. He's respectful of his opponents. You don't find too many people who represent life, whether it be business or sports, like this guy.
"[Roger] has been an ambassador on the court, and an ambassador off the court. How can you be much better than Roger Federer? He's just a credit to the tour. He's a credit to his foundation. So it'll be awful tough to replace Roger Federer."
"The sport is very lucky to have had a Roger Federer."
While Federer's future may seem uncertain, Bollettieri said that of Serena Williams is entirely in her own hands.
The world No. 1 can match Federer's grand slam haul if she retains her U.S. Open title at Flushing Meadows. She also begins her campaign Monday, along with older sister Venus.
Turning 32 on September 26, Serena will be the oldest female winner of the hard-court tournament if she does triumph.
The American won the French Open for the second time in June, but surprisingly lost in the last 16 at Wimbledon. Although she bounced back from that with titles in Bastad and Toronto, a defeat in the Cincinnati final to second-ranked Victoria Azarenka -- just her fourth in 64 matches this year -- again showed that she is not invincible.
"When Serena wants to play the game and she's happy within herself, to me she's the best player in the history of the game of tennis," said Bollettieri, who has worked with both the Williams sisters.
"When Serena's taking the ball early, she's dangerous. When she moves back behind the baseline, she's vulnerable.
"It's all up to Serena. She's won a lot of money, she's done great things for the sport, but it's what she has inside her -- does she still want to compete?"
Williams' bid for a fifth New York crown has been made slightly easier by the withdrawal of Maria Sharapova due to shoulder problems, though she has dominated the Russian in big matches.
Bollettieri said Sharapova, who came to his famed Florida academy as a young girl, will always have weaknesses in her game since the shoulder surgery in 2008 that almost ended her career.
"When you have a shoulder operation, that affects the forehand and it affects the serve," he said.
"When you have a shoulder injury and you cannot serve big-time, you're in trouble because the returns today on the tour are dangerous.
"The girls are standing on the baseline -- if you have a tentative serve they're going to put you to sleep or put you on the defense."