In the 108th minute of play, Hany Mukhtar made a nice move to find enough space amidst three Toronto defenders to fire a shot. Goalkeeper Quentin Westberg made the save, but Rios was there to tap in the rebound for the breakthrough goal.
It was the fifth goal of 2020 for Rios, who entered the game as a substitute in the 71st minute. All five of the rookie striker's scores have come within a 10-game stretch during the regular season and postseason.
Seventh-seeded Nashville SC continued their surprising playoff run in their first season of existence by upsetting second-seeded Toronto FC. Nashville advances to visit the third-seeded Columbus Crew in the Eastern Conference semifinals on Sunday.
Tuesday's game was the type of low-scoring affair familiar to (and favored by) Nashville SC, who had the third-fewest goals allowed and goals scored during the MLS regular season.
Toronto FC had 61.6 percent of the possession but got outshot by a 21-12 margin, rarely showing the form that carried them to the second-best record in the league. Their best chance came in the 26th minute, as a nice passing play set up Nick DeLeon for a close-range shot that was stopped by goalkeeper Joe Willis.
Willis made five saves for his 11th shutout in the regular season and playoffs combined. Willis and Nashville SC as a whole haven't conceded a goal over 210 minutes of postseason soccer.
Despite Nashville's big edge in shots, only two of their attempts in regular time were on net. Multiple quality chances were sent off-target by the expansion side, prolonging the game and giving Toronto FC added life.
Three would-be Nashville goals from Mukhtar and (on two occasions) Jhonder Cadiz were erased by offsides calls.
Toronto FC took their first playoff defeat outside of the MLS Cup final since 2015. They were in three of the past four championship games, winning the title in 2017.