Before he turned in super-human performances at the Beijing Olympics, Bolt was quite pedestrian at the Athens Games. He was eliminated in the heats of the 200 meters, running a 21.05 (with a snail-like .254 reaction time) while hampered by a leg injury. Four years later, Bolt ran a world-record 19.30.
2 of 20Eric Gaillard/AP
She's now Campbell-Brown, but before marriage the two-time Olympic 200-meter champion won her first medal at the Sydney Games, a silver in the 4x100 relay (left, handing off to Beverly McDonald). The 40-year-old anchor on that Jamaican relay, Merlene Ottey, competed in her first Olympics two years before Campbell-Brown, then 18, was born.
3 of 20Timothy Clary/AFP/Getty Images
Cavic, famously out-touched by Michael Phelps in the 2008 100-meter butterfly, actually made his Olympic debut in 2000 at age 16, but good luck finding photos from his two Sydney races (one DQ, one seventh-place finish in a heat). He was a bit better in 2004, reaching the semifinals of the 100 fly.
4 of 20Michael Steele/Getty Images
The Somalian-born, Oregon-trained distance man bowed out in the 5,000-meter heats in Beijing. Now he's a world champion and has his eyes on a 5,000-10,000 double in London, which could make him the host nation star of these Games.
5 of 20Amy Sancetta/AP
Federer placed fourth at the Olympics, one year before he beat Pete Sampras at Wimbledon and three years before his first major title. Federer, then 19, lost the bronze-medal match to France's Arnaud Di Pasquale, who never got past the fourth round of a major. Federer beat Tommy Haas on his way to the semifinals.
6 of 20Peter Read Miller/SI
Hardee was in fourth place in his debut Olympic decathlon before no-heighting in the pole vault (the same event that did in Dan O'Brien at the 1992 Olympic trials). Hardee rebounded to win the 2009 and 2011 world championships, but he's now looking up at trials champion and world-record holder Ashton Eaton.
7 of 20Timothy Clary/AFP/Getty Images
The three-time WNBA MVP was a member of the Canberra Capitals of the WNBL in Australia when she competed in her first Olympics as a 19-year-old. Jackson had 20 points and 13 rebounds in the gold-medal game, a 76-54 loss to the U.S. Jackson and the Aussies are likely looking at a fourth straight silver medal at the London Olympics.
8 of 20Stu Forster/Getty Images
Kitajima swept the breaststrokes at the 2004 and 2008 Olympics and could do the same in London. He was 17 at the 2000 Olympics, fourth in the 100 breast and one hundredth of a second shy of making the semifinals in the 200 breast.
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Lagat, set to compete in his second Olympics as an American, suited up for Kenya in 2000 and 2004. In 2000, he took bronze in the 1,500 behind Kenyan teammate Noah Ngeny, who upset world-record holder and Moroccan legend Hicham El Guerrouj (right).
10 of 20David Longstreath/AP
Lezak (left) was strictly a relay racer in his first Olympics in Sydney, taking silver in the famous 4x100-meter freestyle relay won by the air-guitar-playing Aussies. Lezak got a gold medal in the 4x100 medley relay for swimming in the heats. Now closing out his Olympic career, Lezak, 36, will again be limited to relay duty in London.
11 of 20Mark J. Terrill/AP
Lochte won gold and silver in his first Olympics, both in races involving Michael Phelps. He got the gold first as part of the 4x200 free relay team, opening up a body-length lead on Michael Klim on his leg, and came in second to Phelps in the 200 individual medley. Lochte said his big mistake at the Athens Games was eating three meals a day at McDonald's.
12 of 20Kevork Djansezian/AP
Marta won FIFA Player of the Year honors five straight years. But that came two years after she made her first Olympic appearance. She scored three goals in the 2004 tournament as an 18-year-old, helping Brazil to silver. Marta and Brazil suffered the same result in 2008, second again to the U.S., and also were eliminated by the Americans at the 2011 World Cup.
13 of 20Rick Bowmer/AP
May (now May-Treanor) partnered not with Kerri Walsh, but with Holly McPeak at the 2000 Olympics. They were favored to win gold but lost in the quarterfinals to a Brazilian pair. May-Treanor and Walsh are going for a third straight beach volleyball gold in London, but the favorites are from Brazil.
14 of 20Bill Frakes/SI
Montsho has been one of track and field's sensations since the Beijing Olympics. A late bloomer, she won the world title in the 400 meters last year at age 28 and could win Botswana's first Olympic medal. It will be her third Olympics. In 2004, she was eliminated in the first round of the 400. In 2008, she reached the final and placed eighth.
15 of 20David Longstreath/Doug Mills/AP
Phelps made his first Olympic team at age 15 and took fifth in the 200-meter butterfly, an event he would win easily at the 2004 and 2008 Olympics. He was the youngest U.S. swimmer at the Sydney Olympics. That was the only race in his Olympic career where he failed to medal.
16 of 20Anja Niedrignhaus/AP
Powell (right) was thought to be a medal threat in this first Olympics but placed fifth in the 100 meters in a premonition of future head-scratching major championship disappointments. Powell made headlines in 2004 for looking left and right in his first round and quarterfinal and throwing down the fastest semifinal time (9.95) before falling short in the final. He qualified for the 200 final in Athens but pulled out with a hamstring injury. Powell, entering his third Olympics, has yet to win an individual Olympic medal.
17 of 20Ben Curtis/AP
Richards was a world-class 400-meter runner long before marrying NFL cornerback Aaron Ross. Her Olympic debut came in Athens, where she placed sixth in the 400 at age 19. She also ran the third leg of the gold medal-winning 4x400 relay. In London, Richards-Ross will run the 200 and the 400, looking for her first individual Olympic gold.
18 of 20Heinz Kluetmeier/SI
Schmitt could win four gold medals in London to add to a bronze medal from the Beijing Olympics. She competed in the 200 free and the 4x200 free relay as an 18-year-old in 2008, getting ninth in the former (.01 out of making a swim-off for the final) and medaling in the latter.
19 of 20Simon Bruty/SI
The youngest competitor at the 2000 Olympic trials (12), Vollmer made the Olympics as a rising high school junior four years later and won a gold medal as part of the 4x200 free relay and placed sixth in the individual 200 free. She's become the best 100-meter butterfly swimmer in the world and will be favored to win gold in London.
20 of 20Jed Jacobsohn/Getty Images
Walsh is going to her third straight Olympics as a beach volleyball player in London. But her first Games appearance came indoors. Walsh and the U.S. women were fourth in Sydney.
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