The two teams head to the Maracana having won their last three games in all competitions, scoring 18 goals between them in the process.
After finishing fifth in the Campeonato Brasileiro Serie A last year, Fluminense qualified for this season's Copa Libertadores for the first time since 2013.
The Tricolor Machine have been underachieving in recent years, finishing below 11th place in each of the last four top-flight seasons but now they can look forward to competing in South America's elite competition with a challenging opening fixture on Thursday.
Since March, Fluminense have been competing in the Campeonato Carioca, an annual competition for clubs from the state of Rio de Janeiro. Roger Machado's side currently sit third in the table after winning six of their opening 10 matches, including a 1-0 victory over Botafogo last weekend.
Fluminense have not lifted any silverware since 2012 and have only ever reached the final of the Copa Libertadores once before in 2008, losing on penalties to Ecuadorian side Liga Deportiva Universitaria Quito.
Victory on Thursday is unlikely for Machado's men and any positive result claimed would be a huge boost in their quest to progress from Group D.
River Plate will take part in their 37th Copa Libertadores this year, but once again failed to win the Argentine Primera Division last season, finishing second and three points behind champions Boca Juniors.
The Millionaires have won 36 top-flight titles, more than any other Argentinian side, yet remarkably they have finished at the summit only twice in the last 28 years, with their most recent triumph back in 2014.
They have, however, had much more success in the Copa Libertadores in recent years and as desperate as they are to win their domestic top-flight title this season, success in South America's elite competition is also high on the agenda once again.
River Plate have lifted the Copa Libertadores trophy four times in their history, three fewer than rivals Independiente who have won the most with seven. The Millionaires also sit top of the all-time league table for the competition, having won 180 of their 364 matches – more than any other club.
Their most recent triumph in the competition was in 2018, when they beat bitter rivals Boca Juniors 5-3 on aggregate in the final. Since then, they lost the 2019 final against Flamengo and were eliminated in the semi-finals last season, losing 3-2 on aggregate to eventual champions Palmeiras.
River Plate now face another Brazilian side in the form of Fluminense on Thursday and as one of this year's favourites to win the competition, Marcelo Gallardo's side will fancy their chances of getting off to a strong start and claiming all three points at the Maracana.
Fluminense have no injury concerns heading into Thursday's match and Machado could name the same starting lineup that won last weekend again Botafogo.
Former Paris Saint-Germain and West Ham United forward Nene, who turns 40 in three months' time, is set to start in attack alongside 37-year-old striker Fred and 17-year-old winger Kayky, who has been heavily linked with a move to Manchester City.
Midfielder Agustin Palavecino is likely to start ahead of club captain Leonardo Ponzio – who has only featured three times so far this campaign – while former Benfica and Valencia man Enzo Perez could also start in the middle of the pitch.
Fluminense possible starting lineup:
M. Felipe; Calegari, Nino, Claro, Egidio; Wellington, Y. Felipe, Martinelli; Kakay, Fred, Nene
River Plate possible starting lineup:
Armani; Montiel, Diaz, Martinez, Angileri; Palavecino, Perez, Carrascal; Girotti, Borre, Alvarez
We say: Fluminense 1-3 River Plate
Fluminense have kept five clean sheets in their last eight matches, but their chances of keeping out River Plate, who have scored 16 goals in their last six matches, seem slim.
We feel that the Argentines, who are the favourites for this clash, will be too strong for the hosts and should be able to claim all three points at the Maracana.