Rivalries can define sports, athletes, eras.

That is likely to prove the case when the World Championships begin in Budapest on Saturday, as athletics’ biggest names and rising stars battle for global supremacy.

There will be no shortage of gripping contests to add extra intrigue across the 10 days of competition.

BBC Sport takes a look at the key battles.

Keely Hodgkinson v Athing Mu v Mary Moraa (women’s 800m)

Keely Hodgkinson arrives in Budapest as one of Great Britain’s strongest gold medal hopes.

Last year, in a thrilling race in Eugene, the Olympic 800m silver medallist missed out on the world title by eight hundredths of a second to American Athing Mu – who also beat her to gold at the Tokyo Games in 2021.

With both athletes still just 21, their rivalry could define the event for years to come.

This season, Hodgkinson has defended her European indoor title and set a new British record time of one minute 55.77 seconds among her successes.

It has been a rather different year for Mu, who did not compete for 11 months following her world gold.

She finally began her season eight weeks before this year’s championships, taking victory in 1:58.73 at the New York Grand Prix, but that remains her only 800m in 2023 after choosing to instead race over 1500m at the US trials.

The only athlete to beat Hodgkinson this year is Kenya’s Mary Moraa, who also denied the Briton Commonwealth gold last summer.

The 23-year-old world bronze medallist’s erratic style of racing, varying her pace to frustrate her rivals, will be a significant challenge – as proven at the Lausanne Diamond League in June.

Fred Kerley v Marcell Jacobs (men’s 100m)

Is the 100m head-to-head that fans have waited to see all year finally about to happen?

World champion Fred Kerley and Olympic gold medallist Marcell Jacobs fuelled excitement over a burgeoning rivalry – and an unprecedented one-on-one race – as they exchanged messages on social media earlier this year.

Italian Jacobs, 28, has seen his career disrupted by injuries since his shock Olympic victory in 2021.

He was not present in Eugene when Tokyo runner-up Kerley led a historic American clean sweep of the medals on home soil.

Kerley has played on his rival’s limited racing schedule in offering Jacobs a showdown, suggesting he did not expect his invitation to be accepted.

Jacobs, a long jumper until 2019, responded by sharing an image of his Olympic victory, accompanied by the words: “Whenever you want, wherever you want. But remember that when it mattered more it ended like this.”

While Kerley stands third in the 2023 rankings with a 9.88s performance, behind only Britain’s Zharnel Hughes and Kenyan Ferdinand Omanyala, Jacobs has had another injury-hampered season.

The Italian managed just one race outdoors – an underwhelming seventh-place finish in 10.21 in June – before being sidelined by a back problem, leaving his fitness and medal chances unknown heading into Budapest.

Shericka Jackson v Sha’Carri Richardson (women’s 100m)

In the women’s 100m, Shericka Jackson and Sha’Carri Richardson, the two fastest athletes in the world this year, will begin as favourites in another wide-open sprint event.

Jackson, 29, is aiming to upgrade the silver medal she earned last year behind Shelly-Ann Fraser-Pryce.

She set the fastest time this season when she ran 10.65 in Kingston in July, while she is also the second-fastest over 200m, behind American Gabrielle Thomas.

However, she was beaten by Richardson at the Silesia Diamond League before placing third in London behind Marie-Josee Ta Lou of the Ivory Coast and Britain’s Dina Asher-Smith.

Richardson, with a season-best 10.71, has run four of the six fastest times this year and went unbeaten over 100m until finishing second to Julien Alfred of Saint Lucia in Hungary in mid-July.

The US champion, who missed the Tokyo Olympics because of an anti-doping rule violation for the use of cannabis, continues to remind people: “I’m not back, I’m better.”

Victory in Budapest would be an emphatic way of confirming that.

Fraser-Pryce, Ta Lou, Saint Lucia’s Julien Alfred and Asher-Smith are among several other athletes with podium ambitions and all have run 10.85s or faster this year.

Noah Lyles v Erriyon Knighton (men’s 200m)

Few appear capable of denying Noah Lyles a third successive world 200m title.

The 26-year-old continues to close in on Usain Bolt’s long-standing world record of 19.19 set in 2009 after becoming the fastest American of all-time over the distance when he clocked 19.31 for gold in Eugene last year.

In taking bronze, at the age of 18, rising star Erriyon Knighton claimed his first major medal four years on from switching to athletics from American football.

There was a cold exchange between the pair at last year’s US trials, after Lyles turned and gestured towards Knighton as he crossed the line a mere two hundredths of a second ahead of his compatriot.

In the post-race interview, Knighton did not stick around for long after listening to Lyles discuss his victory, stating he was “not finished” before walking off.

While Lyles did prevail again at the Worlds, Knighton, now 19, is already the fifth fastest athlete of all-time over 200m.

With an additional year of experience and conditioning behind him, his best of 19.72 secs ranks third this year, behind Lyles and Botswana’s Letsile Tebogo, as he aims to move up the podium.

Perhaps the clearest measure of Knighton’s potential is the fact sprinting legend and eight-time Olympic gold medallist Bolt did not go faster than the American’s current Under-20 world record – 19.49 secs – until he was three years older.