Beijing - Usain Bolt returns to the site of his first global triumph this week for a world championships showdown with a convicted dope cheat that has ramifications for athletics beyond just establishing who is the fastest man in the world.
The Jamaican's battle over 100m and 200m with Justin Gatlin will be the highlight of the Aug. 22-30 championships at the Bird's Nest stadium, and victory for the in-form American would serve as an unwelcome reminder of the scourge of doping.
Also read: Bolt back at Bird's Nest for world championships
The governing International Association of Athletics Federations (IAAF) has endured three weeks of embarrassing leaks and accusations that it has neglected its duty to root out drug cheats.
On Sunday, the IAAF denied media reports it had suppressed a 2011 survey that revealed up to a third of the world's top competitors admitted using banned performance-enhancing techniques.
Britain's Sebastian Coe, elected to run international athletics on Wednesday, has promised to set up an independent anti-doping body for the sport, a theme he campaigned on.
With less than a year to go until the Rio de Janeiro Olympics, Bolt needs to dispel concerns over his form and fitness in a city where he won three gold medals in world record times, the defining performance of the 2008 Games.
Chief among the threats to Bolt's dominance is Gatlin, the 2004 Olympic and 2005 world 100m champion who has lost five years of his career to drugs bans.
Undefeated over 100m and 200m since 2013 and boasting the best times of the year (9.74 and 19.57), the 33-year-old Gatlin has comfortably beaten all-comers this year but has yet to face Bolt.
Bolt, who will be 29 on Friday, ran 9.87 in the 100m in London last month and has a track record of saving his best performances for the biggest events.
Another athlete who has produced his best in the big races at the last two global championships is Olympic and world 5,000m and 10,000m champion Mo Farah.
The Briton's preparation for the defence of his titles in Beijing has been disrupted by a storm surrounding doping claims against his coach Alberto Salazar.
Farah has not been accused of wrongdoing but the furore caused him to pull out of a race in Birmingham saying he was "emotionally and physically drained".
Another bid for double gold will come when Ethiopian Genzebe Dibaba bids to add to her family's impressive collection of major titles in the women's 1,500m and 5,000m.
The 24-year-old, sister of three-times Olympic champion Tirunesh and cousin of twice Olympic champion Derartu Tulu, ran a world record of three minutes 50.07 in July, six seconds faster than any of her 1,500m rivals have managed this year.
The event schedule has prevented American Allyson Felix from running for gold in two events and the Olympic 200m champion will compete only in the 400m in Beijing, preferring the greater challenge of the longer event.
Shelly-Ann Fraser-Price has sent out mixed signals over whether she will defend both her 100m and 200m world titles, telling Reuters last week she was dropping the longer event after being listed in the Jamaica team as running both.
In the middle distance events, Olympic champion and world record holder David Rudisha has failed to return to his best form in the 800m since a long injury lay-off, giving Botswana's Nijel Amos hope of a first major title.
Olympic heptathlon champion Jessica Ennis-Hill is also back after a long lay-off, in her case to have a child, and she will be out to reassert her position as Britain's number one over Katarina Johnson-Thompson.
Canada's Brianne Theisen-Eaton is the favourite for gold, however, and if, as expected, American Ashton Eaton wins the decathlon, they will become the first married couple to sweep the two multi-discipline titles.
Few in Beijing can compare to Caterine Ibarguen when it comes to dominance and the Colombian will defend her triple jump title on the back of a 28-competition unbeaten stretching back to her second place at the London Olympics.
Injured New Zealand shot-putter Valerie Adams, who won the last four world titles, is probably the most notable absentee from the championships having lost for the first time in 57 events in June.
Spanish race-walker Jesus Angel Garcia is unlikely to win a medal but will earn his own place in the record books when, at the age of 45, he competes in his 12th consecutive world championships. He won gold in the 50km at his first in 1993.