The match at Krestovsky Stadium in the Russian city of St. Petersburg saw 10-man Poland failing to impress in European play after a disappointing 2018 World Cup in Russia.
Poland, captained by star striker Robert Lewandowski, conceded an own goal by keeper Wojciech Szczęsny in the 18th minute.
Midfielder Karol Linetty equalized early after halftime, making amends for his unconvincing performance in the first half of the match.
Midfielder Karol Linetty scores for Poland against Slovakia in St. Petersburg on Monday. Photo: EPA/Anton Vaganov
But Milan Skriniar made it 2-1 for Slovakia soon after Poland's defensive midfielder Grzegorz Krychowiak was sent off with a red card in the 69th minute, leaving his team a man down and struggling to carve out any real chances.
Romanian referee Ovidin Hategan shows a red card to Poland defensive midfielder Grzegorz Krychowiak (wearing jersey number 10) in the 69th minute. Photo: EPA/Anton Vaganov
The Polish side went into the match missing several key players, including strikers Krzysztof Piątek and Arkadiusz Milik, both of whom had been ruled out of Euro 2020 by injuries.
Poland are ranked 21st worldwide by football's governing body FIFA, while Slovakia are No. 36 in the FIFA table.
The championships, held by the European football governing body UEFA, opened on June 11 after being rescheduled from last year due to the coronavirus pandemic.
As Poland entered the fray, officials said the team’s opening fixture against Slovakia could prove to be crucial to the Poles’ success in the tournament.
After their painful loss to Slovakia, the Poles, under Portuguese manager Paulo Sousa, will next take on Spain as part of their Euro 2020 group-stage campaign on June 19, and then play Sweden four days later.
Poland players with manager Paulo Sousa (centre) during a training session in the Baltic city of Gdańsk last week. Photo: PAP/Adam Warżawa
The 24-team Euro 2020 tournament runs until July 11 in 11 cities around Europe.
Poland in 2016 made it through to the quarterfinals of the European championships in France, losing to Portugal on penalties.
The Polish team suffered an early exit from the 2018 World Cup in Russia.
Source: TVP Sport, IAR, PAP