Eliud Kipchoge of Kenya wins the London Marathon last year. He's the favorite in the men's marathon in Rio.
The Kenyans vs. the Ethiopians.
Like many distance events at the Rio Games, and Olympics past, it often boils down to a race featuring the indefatigable runners from these two African neighbors.
And that's a likely scenario in the women's marathon this Sunday, and the men's next Sunday, the final day of the Summer Olympics.
In every men's and women's marathon since 1996, at least one Kenyan or Ethiopian has made it to the medal stand, with one exception. And that exception proves the rule.
At the 2004 Games in Athens, no Kenyan or Ethiopian man made the podium in the marathon, but the silver medalist was American Meb Keflezighi, who was born in Ethiopia in 1975 (His birthplace of Asmara is now the capital of Eritrea, which broke away from Ethiopia in 1993, after he and his family had left as refugees).
Helah Kiprop of Kenya, Eunice Jepkirui Kirwa of Bahrain and Mare Dibaba of Ethiopia enter a cooling station during the Women's Marathon final during day nine of the 15th IAAF World Athletics Championships Beijing 2015 at Beijing National Stadium on August 30, 2015 in Beijing, China.
Andy Lyons/Getty Images
The Women's Marathon
In the streets of Rio on Sunday morning, Kenyans Helah Kiprop and Jemima Sumgong lead their team. Kiprop won the Tokyo Marathon in February and Sumgong took the London Marathon.
The top-rated Ethiopian is Mare Dibaba, who won the World Championship Marathon in Beijing last summer.
The women's marathon is a relatively new Olympic event. It wasn't contested until 1984 when U.S. runner Joan Benoit ran to glory. She remains the only American woman to ever win a marathon.
Cragg won the U.S. Olympic Trials Marathon in February, with Linden second and Flanagan third. For Cragg, that race was redemption. She just missed the team in 2012 when she finished fourth in the trials.
Linden made the U.S. team for the 2012 Olympic Marathon in London, but did not finish the race due to an injury. Flanagan finished 10th in London. Olympic marathons tend to be slower and more tactical races, so the American runners could also be contenders on Sunday.
The Men's Marathon
In the men's race on Aug. 21, the favorite has to be Kenya's Eliud Kipchoge. He's been just about unbeatable on the world stage in the last two years.
He has the world's fastest marathon time this year, winning the London Marathon in 2 hours, 3 minutes and 5 seconds.
And, not surprisingly, his toughest competition could well come from his teammateStanley Biwott, who won the New York Marathon last year and can in second to Kipchoge in London this year.
The top Ethiopian runner is Tesfaye Abera, who ran a 2:04:24 marathon to win in Dubai in January.
And two other contenders come from the same neighborhood: Eritrea's Ghirmay Ghebreslassie, the surprise winner of the 2015 World Championship Marathon in China, and Uganda's Stephen Kiprotich, the reigning Olympic champion.
The Americans, meanwhile, have an impressive team that could also be a factor.
Keflezighi — the silver medal winner back in 2004, and the fourth place finisher in 2012 — is back again at age 41.
Galen Rupp, 30, won a silver medal in London at 10,000 meters and took the the U.S. Olympic Trials Marathon in February — which was his first ever marathon.
Both the men's and the women's marathons start and finish at Rio's famous Sambodromo, the parade area built for Carnival. What happens in between will determine who stands on the podium this Sunday and next.