"Karen watching Karen" - Amanda Seyfried, Trevor Noah, and Kal Penn watch Karen Khachanov defeat Nick Kyrgios in US Open quarterfinal

Karen Khachanov came up with a brilliant display against Nick Kyrgios to reach a Grand Slam semifinal for the first time in his career at the US Open.

Kyrgios, who was touted as the favorite to win the New York Major after getting past defending champion Daniil Medvedev earlier in the competition, put up stiff resistance but failed to prevent the Russian from ending up as the victor in a five-set thriller.

The engaging contest was witnessed by a host of celebrities, including Amanda Seyfried, Trevor Noah, and Kal Penn, among others.

Seyfried, who played Karen Smith in the 2004 teen comedy Mean Girls, was spotted waving from the stands,

"You can't sit with us," one of the dialogues that best characterized the movie Mean Girls, was aptly amended as the caption for one of Seyfried's photographs.

"Karen watching Karen," the US Open account tweeted.

"You can* sit with us," the tweed read.

Fans reacted to Seyfried's presence with witty tweets in remembrance of the movie.

South African comedian and television personality Trevor Noah was also in attendance in New York and the US Open's Twitter handle did not miss the moment.

"We see you Trevor Noah," the tweet said.

American author and former White House staff member for the Barack Obama administration, Kal Penn was spotted as he looked on animatedly from the stands.

"Kal Penn watching this match," another tweet read.

Harold and Kumar fans were quick to add their thoughts, with one mentioning the 2004 comedy Harold and Kumar Go To White Castle which featured Kalpen Suresh Modi, better known as Kal Penn.

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US Open fans get their money's worth as Nick Kyrgios and Karen Khachanov battle it out in big-serving five-setter

Karen Khachanov celebrates after winning a point against Nick Kyrgios.

US Open fans relished a big-serving quarterfinal, with Karen Khachanov firing 30 aces, just one short of Nick Kyrgios' tally.

The Aussie, who was bothered by his left thigh during the course of the encounter, muttered to himself and threw down a water bottle midway through the fourth set before regaining control. However, he eventually went down 7-5, 4-6, 7-5 6-7(3), 6-4.

After smashing a couple of his racquets after the match, Kyrgios said she was devastated by the result in the post-match press conference.

"I am obviously devastated but all credit to Karen. He is a figher, he is a warrior. I thought he served really good today, the best-ever I played this tournament," a visibly-distraught Kyrgios said.

An elated Khachanov appeared to take a dig at the New York crowd during the on-court interview.

The Russian thanked the crowd for finally "giving him some love" when they cheered for him during his chat after the win.

"I did it, I did it. Now you're giving me some love. Thank you guys, thank you," said the Russian to the crowd who appeared to back Kyrgios as the contest progressed.

Khachanov will face Casper Ruud in Saturday's semifinal.