They are the two indelible images of the 2008 Beijing Olympics - celebration after celebration of gold medals and world records by the American swimmer and the Jamaican sprinter. Because of what they did four years ago, all eyes will be on them when they return to their sports' biggest stages during the London Games, which open July 27.
Unbeatable then, they're not expected to overwhelm the rest of the world - and the clock - quite the same way this time. And each one likely faces a strong challenge from a teammate.
Phelps, who eclipsed Mark Spitz's record total with eight gold medals, now must worry about being faster than Ryan Lochte.
Bolt, the first man to break world records while winning the 100 and 200 meters and 400 relay at the same Olympic track meet, must be wary of what times Yohan Blake will produce.
"I actually think it's a very similar parallel between Phelps and Bolt because I feel like the competition is closer. It's not a question of whether they're not going to do well at the Olympic Games. They are. It's a question of whether the pack has gotten a lot closer to both of them," said Ato Boldon, a sprinter who won four medals for Trinidad and Tobago over two Summer Games and will be part of NBC's broadcasts from the 80,000-seat Olympic Stadium.
Lochte repeatedly has said he feels these Olympics are "my time," and at last year's world championships showed he wasn't intimidated by Phelps, beating him twice in head-to-head meets. Then again, Phelps figures to be in better form at London, and he sent an undeniable message to his biggest rival by beating him in three out of four events at the U.S. trials last month.
"We both hate to lose to one another," Phelps said. "Every time I do get in the water, no matter what stroke it is, he does bring everything out of me and I think that's something that I haven't had with too many competitors throughout my career."
One thing Phelps won't attempt in London: winning another eight gold medals. After qualifying for the same events he competed in at the last two Olympics, he dropped the 200 freestyle. That leaves Phelps with two races against Lochte: the 200 and 400 individual medleys.
"Four years ago, we were trying to literally do everything," Phelps said. "At this point it's, 'Let's go out. Let's have some fun. Let's relax a little bit.'"
Bolt is looking for another three-peat, but a bit of the aura has faded.
"He's not, in a lot of people's eyes, going in as an overwhelming favorite. So how does he respond to that?" Boldon said. "You're tested when you're really challenged, and he wasn't really challenged in Beijing. Pretty much every one of his races was a blowout."
That is not at all an exaggeration.
When Bolt's coming-out party started at those Olympics with a world record of 9.69 seconds in the 100, he won by a huge 0.20 - a huge two-tenths of a second - leaving a gap of several feet between himself and the rest of the field at the finish line. Didn't matter that one of his gold-colored spikes had an untied lace.
And it didn't matter that Bolt even lost some time by mugging for the cameras with about 20 meters to go, stretching his arms out with palms up, then pounding his chest. Well, it mattered to one person - International Olympic Committee President Jacques Rogge, who chastised the sprinter for not showing opponents enough respect
Not content with that, the 6-foot-5 sprinter used his loping strides to follow up four days later with a record 19.30 in the 200, winning by 0.66. He capped it all off by running the third leg for Jamaica's nearly full-second victory in the sprint relay.
"He fascinates me. That's the way I can put it," said Allyson Felix of the U.S., who won silvers in the 200 at the Athens and Beijing Olympics. "I've definitely seen sprinters at the top of their game, but I've never seen anything like that."
But in Beijing, Bolt never saw anyone like Blake.
They're training partners, and they set up quite a story line for London with what happened at the Jamaican Olympic trials, where Blake beat Bolt in the 100 and 200 finals.
Boldon points to others who complicate matters for Bolt. There's Tyson Gay. And Justin Gatlin. And another Jamaican, Asafa Powell, who has yet to win an individual Olympic medal and knows, at age 29, that time is running out.
"This is almost certainly his last shot at being the gold medalist," Boldon said, "and even a broken clock is right twice a day."
Bolt does seem to appreciate what is at stake.
Perhaps mindful of how Phelps' spectacular showing took plenty of attention away from his own feats, Bolt speaks about wanting to provide a suitable encore to cement his standing in sports history.
"I have a goal. I want to be a legend," Bolt said. "And this Olympics, I think, will be the one to make it (so) because it's in London. It's central. It's where everyone is watching."
He'll turn 26 about a week after these Olympics close, and it's certainly possible he could be back in the starting blocks for the 2016 Games. Phelps, who turned 27 last month, insists this is his last hurrah, the final time he'll compete.
Phelps' performance at the Water Cube - eight gold medals, seven world records - was THE story of the 2008 Summer Games, raising his career total to 14 victories, five more than any other Olympian in any sport. He has 16 medals overall, just a tad short of being the greatest Olympian ever. The record, 18 total medals, belongs to Soviet gymnast Larisa Latynina.
She captured nine golds, five silvers, four bronzes over three games from 1956-64. Phelps should eclipse that mark halfway through his program in London.
While still relatively young - Dana Torres attempted to make another Olympic team at 45, and Janet Evans at 40 - Phelps insists that he's done as soon as his hand touches the wall for the final time in London.
He's spent much of the past year savoring moments from his brilliant career, including his Olympic debut as a 15-year-old at Sydney in 2000. He swan only one event (finishing fifth, by the way), but it set the tone for all the victories to come.
"I was so young and I accomplished one of my goals," he recalled.
Phelps has always relished challenges, latching on to everything from a youthful goal or perceived slight to fuel his objectives. Early on, it was his desire to break Spitz's Olympic record from the 1972 Munich Games. Along the way, Phelps was spurred on by various rivals, whether it was losing to Ian Crocker in the 2005 world championships or showing that his one-hundredth of a second victory over Milorad Cavic at the 2008 Beijing Olympics was no fluke.
Now, he's got Lochte, eager to prove he's the world's greatest swimmer.
"That's something that does make me enjoy racing him," Phelps said. "It's definitely helped because I got kind of annoyed and fed up with getting my butt handed to me every race. In big meets, like last year at worlds, it's not fun to lose. I didn't want it to happen anymore, so I started training."
Don't underestimate Phelps' desire to defeat Lochte.
Or anyone standing in the way, for that matter.
"Yeah," Phelps acknowledged, "I have used it as motivation throughout the year and I'll continue to use it as motivation for the next couple weeks. We're obviously going to be training with each other and next to each other.
"It will be some fun races," he added. "I will say that."
But it won't be a Beijing do-over. Phelps' longtime coach, Bob Bowman, knew his pupil didn't train as hard during the last four years as he did leading up to the past two Olympics. Plus, Phelps is four years older, his body battered by a dozen years of world-class swimming. And, really, what does he have to gain by attempting to tie his own record from 2008?
So coach and swimmer decided to drop an event and wipe out any speculation about another eight gold medals.
But Phelps is likely to swim more events in London that any other swimmer except his 17-year-old teammate, Missy Franklin, who's also set to compete seven times.
"I have a big event program," Phelps pointed out. Dropping an event "just allows me to put my energy elsewhere instead of trying to control it for another three races."
"He's the defending champion, so everyone will be looking for him to come back and do it again. But we have a lot of sprinters coming up to challenge him," Powell said. "So it's not going to be as easy as Beijing.
"Definitely, he has to watch his back."
(For more sports news and updates, follow Ahram Online Sports on Twitter at @AO Sports and on Facebook at AhramOnlineSports)