Former player John Isner sent a strongly worded message to mourn the death of Charlie Kirk, the American political activist, who was shot dead at 31. Kirk was often at the forefront, promoting conservative causes.John Isner, the American professional player who peaked at No. 8 in the ATP rankings, is known for his fastest serve and for recording the most aces in ATP Tour history. He retired in 2023 but remains connected to the sport through the Nothing Major podcast, alongside fellow former American players Jack Sock, Sam Querrey, and Steve Johnson.Isner often shares his views on social media, the most recent being about the Charlie Kirk attack that resulted in the activist's death on the spot. Kirk, a Trump ally and Republican, who often faced criticisms for being a polarizing figure, was addressing the students at Utah Valley University on his “The American Comeback” Tour.The 31-year-old was seated under the tent while answering questions when a gunshot struck him in the neck, leading to profuse bleeding and his death shortly after. The disturbing video of the incident has sent shockwaves around the nation, even garnering John Isner's attention.The Grand Slam semifinalist sided with Kirk's views and mourned the incident, writing:"If you’re not sick to your stomach with what happened in Utah today then you’re seriously f****d in the head. Charlie spoke truth. His platform was about having civil conversations with those he didn’t agree with. For that, he was shot dead. 31 years old. Two young kids. Follower of Christ and Husband. RIP Charlie."The Trump administration announced that American flags would be flown at half-mast for a couple of days to pay respect to Charlie Kirk.John Isner calls out ITF policies that didn't let Russian and Belarusian players represent their flags over war involvementIsner at the 2023 US Open - Day 4 - (Source: Getty)John Isner has never shied away from expressing his opinions on all things tennis and beyond. Just days before the US Open, he criticized the International Tennis Federation’s decision to ban Russian and Belarusian players from competing under their national flags due to the war in Ukraine. The 2022 implementation continues to sideline Russians and Belarusians from the Davis Cup and the Billie Jean King Cup as well.Isner took to his X handle and shared that it was 'lame' and needed to be stopped."Can the Russian tennis players get their flag back? Bit ridiculous now. It’s so lame."Notably, Isner and Nicholas Mahut played the longest tennis match in history, which lasted 11 hours and 5 minutes at the 2010 Wimbledon.