Copa Libertadores semifinals: Boca vs. River ante raised higher; can Gremio beat Flamengo?

Buenos Aires is bracing itself twice over for the events of Tuesday. First, heavy rain is forecast. Second, there could already be enough thunder and lightning supplied by the first leg of the Copa Libertadores semifinal between River Plate and Boca Juniors -- a half anticipated, half dreaded meeting of historic rivals.

Last year, of course, they met in the two-legged final. The whole saga proved too big, too dramatic, too intense for the city, and after fan violence outside River's stadium the decisive second leg was controversially moved to Madrid, where River won 3-1.

There could be no repeat. From this year the final is a one-off clash in a neutral venue -- which this time is Santiago, Chile. And so fate has decreed that next best/worst (delete as appropriate) scenario will apply. The teams meet home and away in the semifinal, this week in River's ground, and three weeks later in Boca's.

The events of last year continue to ripple. Boca feel they should have been awarded the trophy; River resented not being able to play at home. If it were possible, the ante has been raised still higher.

River are the only unbeaten team left in the field, but they have only won three of their 10 matches in this campaign and their total of 13 goals scored is the lowest of the four semifinalists; Boca have scored 17 (the highest) and are also top of the Argentine league.

Still, defending champions River are widely seen as the favourites to reach the final, and as the more expansive, attractive side. In his five-year reign, coach Marcelo Gallardo has shown an ability to develop a rich attacking repertoire, which he hopes will decide the tie.

Boca, meanwhile, are a less expansive side than last year, when they usually operated with two wingers. This year's coach is Gustavo Alfaro -- the bulk of his considerable experience has been acquired in charge of relatively small clubs, and it shows. His Boca side seem happiest on the backfoot and their most impressive displays of the campaign have come away from home in the knockout rounds.

In the group phase Boca did not win on their travels. Since then, though, they produced compact, disciplined performances to win away matches at highly difficult venues -- on the synthetic surface of Athletico Paranaense in Brazil, and at the altitude of Quito against LDU. Alfaro will look for more of the same away to River and it could be that windy and rainy conditions will hamper the hosts more than the visitors.

If the all-Argentine tie is an occasion to be approached with a dose of trepidation, the all-Brazilian clash in the other semifinal holds the promise of an excellent spectacle.

Gremio and Flamengo are best footballing sides in the land. Champions in 2017, Gremio have reached the last four for the third consecutive year. Coach Renato Portaluppi has done a fine job as their budget is less than that of some of the Brazilian clubs, but, year after year, Gremio look to play a highly pleasing possession-based game.

Flamengo, meanwhile, are giants who have taken a while to draw themselves up to their full height. The Rio de Janeiro club are the best supported team in Brazil, with a national fanbase. After years of chaos, they have sorted out their finances and assembled a squad of considerable depth, while the choice to appoint Portuguese coach Jorge Jesus has proved a masterstroke.

Announced in the middle of the year, Jesus has cause a mini revolution, managing to implement a high defensive line that enables the team to stay compact and keep the opposition under pressure. Spanish centre-back Pablo Mari marshals the defence; experienced full-backs Rafinha and Filipe Luis construct from deep; Gerson provides thrust from midfield; the front four -- playmakers Everton Ribeiro and Giorgian De Arrascaeta, plus strikers Gabriel "Gabigol" Barbosa and Bruno Henrique -- are all firing together. In full flow they are a stirring sight, but Gremio trust in their capacity to beat them.

After some time in the wilderness, Luan, Gremio's playmaking hero of the 2017 triumph, is starting to find some form. He could be the provider for the dangerous winger Everton, the breakout star of Brazil's recent Copa America win as Everton's strength (cutting in from the left) is pitted against Flamengo's weakness (defending behind the line down their right.) It is all set up for an intriguing contest.

The brightest fireworks might be reserved for the second leg in the Maracana on Oct. 23. But Wednesday's first leg in Porto Alegre should light the fuse.