The Laver Cup is decided this weekend.
Tennis icon Björn Borg is captain of the European team - which is fighting against "the rest of the world".
On Friday night, Borg and the audience in London witnessed unexpected scenes.
In the middle of the match between Stefanos Tsitsipas and Diego Schwartzman, an activist ran onto the pitch.
He sat down near the net and set fire to a piece of paper.
"Somewhat scary"
The person's arm briefly caught fire before he was carried out by security personnel.
- It's a little scary.
You never know what they (activists) are up to, says commentator Niclas Rodhborn in Eurosport.
The activist's t-shirt read "End UK private jets", which means "stop British private jets".
Shortly after the incident, Borg was interviewed on the broadcast, where the focus was on sports.
- It's good tennis.
I enjoy it.
To be a part of this is fantastic, he says.