Swiss great Roger Federer, winner of a record 16 Grand Slams, including six at Wimbledon, remains a threat at the All England Club despite being 30. “Wimbledon is his best chance to win another major,” said three-time champion John McEnroe. (Joerg Sarbach/Associated Press)

After dominating tennis so completely in his mid-20s, Roger Federer has only one trajectory remaining in a brilliant career: downward, as certain as gravity.

And even that has been graceful.

It’s difficult to pinpoint a skill of Federer’s that has diminished these last two years, in which he slid from No. 1 in the world to No. 3. His movement, at 30, remains fleet and fluid. His serve is shrewdly placed with plenty of pace. And he brandishes a full array of strokes and the smarts to deploy each as circumstances dictate.

When Federer has stumbled of late, it has been to challengers determined to risk more, in the case of Novak Djokovic, or pummel the ball however long it takes to win a battle of attrition, in the case of Rafael Nadal, who now holds an 18-10 record against the Swiss.

The upshot has turned the conversation about who is the best player of his generation into a three-way debate. And if Nadal, 26, and Djokovic, 25, continue amassing Grand Slam titles at the rate they are (Nadal has won five of the last nine majors; Djokovic, the other four), they may one day earn a place alongside Federer and Rod Laver in the debate over the greatest ever.

Before retiring, Federer would love to raise the standard higher still by extending his record 16 major titles beyond the reach of Nadal (11), Djokovic (five) and any phenom to come.

His best opportunity comes at Wimbledon, which gets under way Monday at the All England club, where Djokovic, the defending champion; Nadal, a two-time victor; and Federer, with six titles, are seeded 1-2-3, respectively.

Despite clear evidence that the gap has closed between the Swiss and his younger rivals, Federer remains the pick of three-time Wimbledon champion John McEnroe.

“To me, Wimbledon is his best chance to win another major,” says McEnroe, 53, a three-time Wimbledon champion who will provide commentary for ESPN. “He seems to still want it, as much as he has already won it. He’s a lot better athlete than he’s given credit for. His movement has allowed him to remain incredibly healthy. . . . His record speaks for itself.”

With a seventh Wimbledon title, Federer would equal the record held by his idol, Pete Sampras. But that’s not what drives him, Federer said after being ushered out in last year’s quarterfinals by Jo-Wilfried Tsonga of France. It was a confounding match, in which Federer squandered a two-sets-to-none lead for the first time ever in a Grand Slam.

That uncharacteristic inability to close is what stood out most to Chris Evert, whose Hall of Fame career was defined by a ruthless ability to finish off opponents.

Federer also took a two-sets-to-none lead against Djokovic in the semifinals of last year’s U.S. Open but failed to convert two match points and fell, 7-5, in the fifth.

In that case, it was a bit more difficult to fault Federer. Djokovic blasted the shot of a lifetime to stave off one match point, a forehand cross-court service return that was struck with the force of Zeus and had zero margin for error.

“I never played that way,” Federer said afterward, as if the recklessness of Djokovic’s shot offended his sense of aesthetics and fair play. “For me, this is very hard to understand. How can you play a shot like that on match point?”

With time clearly working against him, Evert predicts Federer will alter his approach at Wimbledon this year — particularly if he meets Djokovic in the semifinals or advances to a fourth final against Nadal. With grass his best surface, Federer should feel emboldened to take more risks, suggests Evert, who returns to ESPN’s broadcast booth for a second year.

“I think he has flair; he has finesse,” said Evert, who won three of her 18 major titles at Wimbledon. “I don’t think he has the patience to sit out there and hit groundstrokes all day.”

The women’s field features its own 30-year-old former No. 1 seeking to reclaim Wimbledon’s crown: four-time champion Serena Williams.

After starting 2012 in impressive form, Williams suffered a stunning first-round defeat at the French Open this month, which dropped her ranking and seeding to No. 6.

But like Federer, she has the strokes and smarts to dominate on grass. And placed in the opposite half of the draw from top-seeded Maria Sharapova, Williams likely represents the biggest hurdle to the Russian’s goal of replicating her 2004 Wimbledon fairy tale, in which she upset top-seeded Williams to win her first major at 17.

Now 25, Sharapova arrives on the heels of a French Open triumph that completed a career Grand Slam, with new confidence and an improving serve.

The women’s field includes two dangerous unseeded players: five-time champion Venus Williams, 32, who has played just 17 matches this year while learning to manage an autoimmune disorder that causes joint pain and fatigue; and former No. 1 Kim Clijsters, 29. who has battled hip and ankle injuries and this past week withdrew from her semifinal match at a grass-court tuneup in the Netherlands because of a stomach muscle strain.