The changing of the guard at Buckingham Palace is one of the most celebrated and stylized rituals in the civilized world. Every other day at 11:30 a.m. local time (summer hours), brass instruments blare, horses parade and the royal sentinels stiffly rotate for another shift.
In tennis, this transition is somewhat less precise.
Still, only a few miles from downtown London at this year's Wimbledon tournament, we might have seen the beginning of a tectonic shift in the men's game:
• Nick Kyrgios, a loosey-goosey 19-year-old Australian who entered the event with all of three ATP-level career victories, won four matches there, culminating with a stunning defeat of No. 1-seeded Rafael Nadal.
"Oh, I don't think the guard is changing, really," the Canadian said earlier this month on his way to the Citi Open title in Washington, D.C. "Djokovic is still Djokovic, Rafa just won the French, Murray won a couple of Slams and Federer is still dangerous. They're the guys who are still winning the Slams.
"But I do think the younger guys are sensing that there are opportunities for us if we can improve our games."
Tennis Channel analyst Justin Gimelstob sees it as a "neat" time in men's tennis.
"It's a convergence where the top players are still viable and the younger players are feeling it," Gimelstob said. "I see it as a special time of star power, and stars on the cusp of breaking through.
"You know it's going to happen -- just not exactly when."
Two for the money
The older we get, the better they were.
Baby boomers love to talk about the era when Bjorn Borg and Jimmy Connors, Ivan Lendl and John McEnroe did battle. Older fans rave about the exploits of Aussies Rod Laver, Ken Rosewall and Roy Emerson.
But, based strictly on the currency of championship titles, this is the golden era of men's tennis. Federer, 33, is the all-time leader with 17 Grand Slam singles titles. Nadal tied Pete Sampras at the French Open for second in the Open era, with 14. Djokovic (seven with Wimbledon) and Murray (two) are still in their prime. Juan Martin del Potro, who carried off the 2009 US Open, might have won more majors if he hadn't suffered two serious wrist injuries.
The Big Four -- Federer, Nadal, Djokovic and now Murray -- have enjoyed a suffocating monopoly on men's tennis. Since Marat Safin won the 2005 Australian Open, they have taken 36 of the 38 Grand Slam titles available, with del Potro and 2014 Australian Open champion Stan Wawrinka being the only exceptions. They are an incredible 20-for-20 at the Euro Slams, those in Paris and London.
"You never know," mused ESPN analyst Brad Gilbert from his home in Northern California, "the Big Four could go for a while longer. But you also know, at some point, there's got to be room for somebody else."
A decade ago, the 23-year-old Federer finished as the No. 1-ranked player in the world. Lleyton Hewitt, also 33, is the only other active player (at No. 41) from that top-10 list. The average age back then was 25.4 years, more than two years younger than it is today.
Raonic and Dimitrov are the only two players under 25 among the top 10 -- and, setting del Potro aside -- they are four years younger than the next-youngest, Djokovic and Murray. They are seen as the immediate heirs to the Big Four.
At 6-foot-5, 198 pounds, Raonic has the classic big-serve, big-forehand game. He has worked hard on his movement and improved incrementally the past several years, from No. 151 in 2010 to No. 31, No. 13, No. 11 and the present No. 6.
"I'm just more consistent now," Raonic told ESPN.com. "I learned at Wimbledon how to manage my game. It comes down to a few points in a close match, and those [top] guys are just so consistent. They know when to go for it.
"I have to hold serve and be more aggressive. I need to find courage to go for it more in the big moments."
The win in Washington, over fellow Canadian Vasek Pospisil, 24, ended an 11-month run without a title and launched him to a career-high No. 6. Raonic reached the quarters in Montreal (losing to Feliciano Lopez) and the semifinals in Cincinnati, falling again to Federer.
"Raonic is a big, strong guy who wins with brute force," said Gimelstob, who called the matches in Washington and is a member of the ATP board of directors. "The commitment to excellence is there ... lack of movement? There's no doubt he needs to work more on his movement, but power is a great eraser.
"There are certain things Raonic has that you can't defend against."
Said Gilbert, "Raonic has the biggest weapons, but Dimitrov [ranked No. 8] has the greatest skill. He has great hands and feet and can comfortably play both offense and defense. He does everything well."
The 6-3, 176-pounder modeled his all-around game on Federer's, which is why he was dubbed "Baby Fed." Gimelstob practiced with Dimitrov -- who may no longer be better known as Maria Sharapova's boyfriend -- this summer in Los Angeles.
"He has a very stylish game," Gimelstob said. "He has the most untapped arsenal, even though he's already accomplished so much. He has a little bit of the Sampras look, the way the ball comes of the racket -- it's so explosive, lively. There's so much growth available in his game."
Dimitrov's first major splash came at the Australian Open, when he beat Raonic in the third round and advanced to the quarterfinals before losing to Nadal. Winning titles on such varied surfaces as Acapulco (hard), Bucharest (clay) and Queen's Club (grass) underlined the diversity of his game. The win over Murray at Wimbledon showed he's ready to step up.
"There are some eerie similarities between his game and Federer's," Gimelstob added. "If you're going to pick someone to emulate, Federer's not a bad choice."
The next wave
Perhaps the best match at Wimbledon involved not two players, but four.
A 21-year-old American named Jack Sock and partner Pospisil took on Bob and Mike Bryan, who had collected 15 more Grand Slam doubles titles than the young pair. Somehow, Sock and Pospisil won their first major title in a rousing five-set final.
"That was awesome," Sock said from his hotel room in Toronto earlier this month. "That was probably the most fun I've had playing a match. To beat the best team ever, on Centre Court ... how can you top that?"
Maybe by following the legacy of fellow Nebraskan Andy Roddick, the last American man to win a Grand Slam title, the 2003 US Open. Sock, who has grown dramatically -- in stature and within his game -- might have the chops to do it one day.
He has had a nice little summer for himself, getting to the semifinals in Newport and Atlanta, which helped earn wild-card berths into the two North American ATP Masters 1000 events. Sock, now ranked No. 56, lost to Raonic in Canada in a match that featured two taut tiebreakers. No. 16 seed Tommy Robredo -- the same guy who would go on to stun Djokovic -- beat him in Cincinnati.
"It's been a good summer, yes," Sock said. "But I need to be more consistent, reach the quarters and semis of more tournaments. And I've got to do better in the majors. I feel like it's something that's coming."
So, too, are a bunch of young would-be contenders. Here are a few to consider:
• David Goffin is a 23-year-old from Belgium who looks 10 years younger. He won his first ATP title earlier this summer in Kitzbuhel and cracked the top 50 for the first time. His opponent in the final was ...
• ... Dominic Thiem, a 20-year-old from Austria who has already won an astounding 17 ATP-level matches this year. He won two qualifying matches in Madrid and took out Australian Open champion Wawrinka on the way to the third round.
• Nick Kyrgios, the 19-year-old Aussie, might have the most upside of any player in this up-and-coming group. He won eight matches at the Challenger in Nottingham and made his wild card count at Wimbledon. There were flashes of the future in his win over Nadal.
• Borna Coric, a 17-year-old Serbian from Zagreb who has patterned his game after -- who else? -- Djokovic. In Umag, the 6-1, 174-pound athlete beat No. 46 Edouard Roger-Vasselin in straight sets. He's already up to No. 193.
• Alexander Zverev, a 17-year-old from Hamburg, Germany, is already No. 157. Playing against the big boys, he won four of five matches in Hamburg last month, beating four players -- including No. 19 Mikhail Youzhny -- ranked among the top 85.
Ask Zverev who his tennis role models are -- and he will name the entire Big Four.
"If I play as well as them one day, I will be happy," Zverev recently told ESPN.com tennis editor Matt Wilansky.
What will it take to beat them?
"I need to get into the top 100 just to talk about them, and then the top 50," Zverev said. "There are a lot of young talented players who tried and failed to even make the top 100. I hope I won't be one of those, but for now I just don't know."
Sock isn't ready to write off the Big Four, but he senses a change in the wind.
"I'm not sure the window is open, exactly," Sock said. "But there might be a crack there."