This is the highest prestige match a Hungarian referee has been in charge of since Sándor Puhl's 1994 FIFA World Cup Final.

Kassai was the head of an all-Hungarian crew in the 2011 UEFA Champions League Final at Wembley Stadium in London, where he cautioned two players apiece on both the Barcelona (Daniel Alves and Victor Valdés) and Manchester United (Antonio Valencia and Michael Carrick) sides.

[4] During the England and Ukraine game a Ukrainian strike was cleared by English defender John Terry, but TV re-plays showed that the ball had crossed the line.

On 18 April 2017, he refereed the 2016–17 UEFA Champions League quarter-finals second-leg between Real Madrid and FC Bayern Munich.

He was criticized for poor refereeing decisions, as Real Madrid and Bayern Munich had scored two respectively one goal in which they were offside.

[12][13] On 7 November 2018, Kassai refereed the 2018–19 UEFA Champions League group stage fixture between Manchester City and Shakhtar Donetsk.

During the game he awarded the home side a penalty when Raheem Sterling unintentionally tripped himself in the box, while there was clearly no contact between him and Shakhtar defenders at all.