Portugal face Spain in the World Cup round of 16 tonight at the Dallas Stadium in Arlington, Texas, with the winner facing either the USA or Belgium in the quarter-finals in Los Angeles on Friday 10 July. It's a rematch of last year's Nations League final, which Portugal won on penalties against the European champions - but the balance of power looks different now. Spain arrive unbeaten and yet to concede a goal at this World Cup, having topped their group before storming past Austria 3-0 in the round of 32, and they're unbeaten in their last 34 matches, one short of their all-time record. Opta's supercomputer gives Spain a 49.2 percent chance of winning in normal time against Portugal's 25.6 percent, with a quarter of simulations going to extra time. Portugal, by contrast, have stuttered their way here. They're unbeaten and have conceded only twice in four games, but looked short of fluency in a 1-1 draw with DR Congo and a 0-0 stalemate with Colombia, and they needed high drama to survive the last 32. Croatia thought they'd forced extra time through Josko Gvardiol, only for ball-sensor technology to confirm a touch from an offside Croatian and rule out the last-gasp equaliser, sealing a 2-1 win. There was history in the group stage too: Portugal won a World Cup match after trailing for the first time since 1966. But the last 16 has been a graveyard for them. Their World Cup runs in 2010 and 2018 both ended at this stage. Roberto Martínez has no fresh injury concerns, and he's not expected to drop Ronaldo from the starting XI despite substituting the 41-year-old against Croatia. Which brings us to the story within the story: the generational duel. Ronaldo, the second-oldest player at the tournament, faces 18-year-old prodigy Lamine Yamal, and while the Portuguese captain has deflected questions about his future, his sister has said he'll retire from international duty when this World Cup ends. Ronaldo has three goals in the tournament and pedigree in this fixture - the sides' most recent World Cup meeting, in 2018, ended 3-3 with a Ronaldo hat-trick - but Spain's danger man has been Mikel Oyarzabal, who has directly contributed to five of Spain's eight goals at this tournament.