Natasha Hastings in the 4x400m final at the IAAF/BTC World Relays Bahamas 2017 (© Getty Images)
The USA has won both the men's and women's 4x400m relays at each of the three previous editions of the IAAF World Relays, and will arrive in Yokohama as the teams to beat.
The US men's squad is solid, but young, with all making their debut at the World Relays. Michael Cherry has been a reliable relay runner in recent years, contributing legs to the silver medal-winning squads at the 2017 World Championships and 2018 World Indoor Championships.
So has Fred Kerley, a teammate on both of those squads, who with 43.70 credentials - the fastest in the field of entrants - and the 2018 Diamond League title under his belt, also brings raw speed to the table.
The pool also includes Kerley's younger brother Mylik, a former NCAA 4x400m relay champion indoors and outdoors, Josephus Lyles, a world U18 and U20 medallist and younger brother of sprint phenom Noah, and Paul Dedewo, a 44.43 400m runner.
Botswana finished second two years ago, edged by just 0.15 in one of the most memorable races of the 2017 edition. Three members of that squad - Isaac Makwala, Onkabetse Nkobolo and Baboloki Thebe - have made the trip to Yokohama looking to settle that score. Thebe, still just 22, already has a pair of African 400m titles to his credit while Makwala, the veteran on the team at 33, raced to last year's Commonwealth title.
Bahamas, the runners-up in 2014 and 2015, are led by Steven Gardiner, the 400m national record holder at 43.87, who has illustrated strong fitness already this season with a 20.04 200m in early April. But he'll have to count on some relative newcomers to come up with big performances to figure in the medal chase.
Don't discount Trinidad and Tobago, the reigning world champions. Machel Cenedio and Jareem Richards, members of the triumphant squad who ended a US World Championships win streak in the event at six in London, will take to the track here.
Others in the mix include Belgium, who raced to bronze in 2015. Their squad includes the Borlée brothers Dylan, Kevin and Jonathan - who were on that 2015 squad - along with Janathan Sacoor, the world U20 champion.
Poland could be a factor. Their pool includes Karol Zalewski and Jakub Krzewina, the lead-off and anchor legs of the gold medal-winning squad at last year's World Indoor Championships.
Unlike in previous men's World Relays finals, which all came down to fierce homestretch battles, the women's were decidedly more lopsided, with US quartets winning by 1.53 seconds in 2014, 3.10 seconds in 2015 and 4.92 seconds in 2017. No one that ran on those victorious squads is back this year, but the squad boasts enough experience to arrive as the team to beat.
Thirty-year-old Jessica Beard has three world 4x400m titles to her credit and clocked 50.18 last year, a career best. Courtney Okolo, a member of the 2016 Olympic champion quartet and a two-time world indoor gold medallist, also has plenty of experience. As does Shakima Wimbley, a world outdoor champion in the event in 2017 and world champion indoors in 2018.
Jamaica, which finished runner-up in 2014 and 2015 and took bronze two years ago, looks set to mount the strongest challenge. Anastasia Le-Roy, the 2018 Commonwealth Games silver medallist, heads a squad that includes two-time Olympic silver medallist Christine Day, her Rio teammate Chrisann Gordon and full lap hurdles standout Janieve Russell who led off the Jamaican quartet two years ago.
But Poland also brings a strong team. Their pool includes Malgorzata Holub-Kowalik, Iga Baumgart-Witan, Patrycja Wyciszkiewicz and Justyna Swiety-Ersetic, the quartet that took the European title last year. Holub-Kowalik, Baumgart-Witan and Swiety-Ersetic made up three-quarters of the team that finished second at the World Relays two years ago.
Nigeria, the bronze medallists at the inaugural edition and consistent top-five finishers at global events, could be a medal threat as well. So could Great Britain and Northern Ireland, the third place finishers in 2015, who more recently were second at the European Indoor Championships.
India and host Japan finished second and third at the Asian Championships in Doha last month and will be targeting top-five finishes here.
The top ten placed teams in both the men's and women's 4x400m will automatically qualify for the IAAF World Athletics Championships Doha 2019.
After its wildly successful debut at this event in Nassau two years ago, the mixed 4x400m relay returns to centre stage here, but minus defending champion Bahamas.
That would suggest that the US, given its prodigious depth in the event, will start as the squad to beat. But with a team that's largely untested internationally entered, the race appears to be wide open.
Nathan Strother, this year's World Indoor Tour champion, is the most experienced. The 23-year-old clocked 44.34 a year ago and finished third at the Continental Cup. On the women's side, Olivia Baker, a medallist at the 2013 and 2014 World Youth and World Junior Championships is the standout.
Conversely, Poland, a perennial 400m power, is considerably more experienced. Its pool features of world indoor 4x400m champions Karol Zalewski and Jakub Krzewina, who are also included in the men’s 4x400m pool, and Patrycja Wyciszkiewicz, a world indoor silver medallist and European outdoor gold medallist in the 4x400 last year.
Others will also be factors.
The Jamaican squad, which finished third in this event two years ago, includes Javon Francis, a 2016 Olympic silver medallist in the event and Tiffany James, the 2016 world U20 400m champion.
Belgium's hopes lie with relay veteran Kevin Borlée, world U20 champion Jonathan Sacoor and this year's European indoor 400m silver medallist Cynthia Bolingo. But Borlée and Sacoor will have to recover quickly: the mixed relay final is scheduled immediately after the men’s 4x400m, where both are also expected to compete.
Bob Ramsak for the IAAF