Wheelchair racers Marcel Hug (SUI) and Tatyana McFadden (USA) will be looking to tighten their grip on the Abbott World Marathon Majors leaderboards when Series X resumes at the Paralympic Games in Rio on Sunday September 18.
The pair dominated the first two races in the inaugural wheelchair series, both athletes winning twice in the space of six days at the Boston Marathon and the Virgin Money London Marathon this April.
With a maximum 50 points apiece, they top their respective standings Hug holding a 25-point lead in the men's contest while McFadden has a cushion of 18 in the women's.
The racers accumulate points from five of the eight races that make up the one-year global competition with a winner-takes-all prize of US$50,000 for the award for the top man and woman overall.
Hug already has the Series X title in his sights, having triumphed in a dramatic three-way finish in Boston ahead of South Africa's Ernst van Dyk and Australia's Kurt Fearnley before winning by just one second from Fearnley in London less than a week later.
The 30-year-old Swiss racer, nicknamed the Silver Bullet due to his eye-catching helmet, won the world marathon title in 2013 a year after claiming Paralympic silver behind Britain's David Weir at London 2012.
Hug's aim is to go one better in the 2016 T54 race at Rio's Fort Copacabana where Fearnley will be looking to regain the Paralympic gold he won in Athens and Beijing after taking bronze four years ago.
The 35-year-old has won 16 races at World Marathon Majors events over the years, was world champion in 2006 and 2011, and lies second in the Series X standings thanks to podium finishes in both London and Boston this year.
Van Dyk will also seek to close the gap on Hug. He lies third on 20 points after finishing runner-up in Boston, where he has won 10 times, and fourth in London just behind Weir, Britain's six-time London Marathon champion.
Weir could only finish third on home soil this year, but the 37-year-old will be keen to defend his Paralympic title knowing that victory could move him up the leaderboard from fourth to second.
James Senbeta will also want to add to his tally. The US racer has two points so far after placing fifth in the first two races.
Other contenders to watch for include Japanese pair Masazumi Soejima and Kota Hokinoue, plus Hug's compatriot Heinz Frei, the 58-year-old who won the Paralympic title in 1984 and 1992 and has held the official world record since 1999.
McFadden is the outstanding favorite for the women's gold after dominating women's marathon racing since 2012.
The 27-year-old US star was ninth at the London Games but has lost just once in four years since the start of 2013, a period in which she has remained unbeaten in Boston, London, Chicago and New York while also winning the world title last year.
McFadden's main challenge should come from Switzerland's 2013 world champion Manuela Sch�r and Japanese veteran Wakako Tsuchida, who placed second and third respectively in both Boston and London this year.
Sch�r lies second in the Series X standings with 32 points and will hope to turn the tables on McFadden after losing in a sprint finish to the American in London. The 31-year-old has also chased McFadden to the line in Chicago and New York in recent years, while she has shared the official world record with Tsuchida since 2013.
Tsuchida won the world title in 2011 and was fifth at the 2012 Paralympics. More significantly, she ended McFadden's 13-race win streak at the Tokyo Marathon earlier this year while claiming her ninth straight title in the Japanese capital.
McFadden's teammates Amanda McGrory and Shirley Reilly will also be looking for points as well as medals McGrory currently lies fourth with five points while Reilly is the defending champion from London 2012.
Zou Lihong is the only other entrant on the current leaderboard. The Chinese racer finished fifth on her marathon debut in London and could be an outside bet for a medal.
Other notable racers looking for their first points of the series include Switzerland's two-time Paralympic bronze medallist, Sandra Graf, and Australia's Christie Dawes who was sixth four years ago.
Both races start at 12:30 Rio time on Sunday Sept. 18.