Cilic sends fourth seed Draper crashing out of Wimbledon
Cilic, the Croatian world number 83, blasted 53 winners in a 6-4, 6-3, 1-6, 6-4 victory that stunned the partisan British fans on Court One on Thursday.
Draper is the new figurehead of British tennis after taking on a role filled for a generation by the now-retired former Wimbledon winner Andy Murray.
But the 23-year-old has a poor record at his home Grand Slam.
He is yet to make it past the second round in four appearances at the grass-court tournament.
"It's probably one of my toughest losses. Pretty frustrated and upset. It hurts badly," Draper said.
"I've been disappointed with my game on grass this year. I felt great on clay and hard courts but as soon as I came on grass I felt a big difference.
"It highlighted a lot of weaknesses in my game, especially against a player as good as him on grass. I can take a lot of lessons from it."
Draper is the latest highly-ranked star to endure a surprise exit from Wimbledon this year, with German men's third seed Alexander Zverev and four of the top five women's seeds all eliminated.
Draper made his breakthrough last season with a run to the US Open semi-finals, yet he has failed to make the last eight at any of this year's three Grand Slams.
"It makes me think that Andy Murray's achievement of winning here twice is just unbelievable," Draper said.
"Obviously you guys mention the pressure all the time but the main reason is I wasn't good enough today.
"Even though I've had an amazing progression in the last 12 months, this shows the work I need to do. I thought I was further ahead than I am."
'My emotions are just incredible'
In contrast to Draper's woes in south-west London, Cilic has an impressive record at Wimbledon, finishing as runner-up in 2017, when he lost in the final against Roger Federer.
The former world number three has not played at the All England Club since 2021 and has slipped down the rankings after battling a knee injury that required surgery.
"My emotions are just incredible. Where I was two years ago, I can't even describe. It has been a long journey but I never lost any faith," Cilic said.
"It was a long and testing period, plus a huge challenge for me in this part of my career to come back and play at this level.
"To play in front of this crowd is incredible. I'm just feeling great and hoping I can continue to play well."
Cilic has reached three Grand Slam finals, winning his sole major title at the 2014 US Open and losing to Federer at Wimbledon in 2017 and the Australian Open in 2018.
Now 36, the big-serving Cilic is firmly in the twilight of his career.
He had lost in the opening round at the Australian and French Opens this year, but his superb display against Draper underlined his enduring quality on grass.
He returned to the top 100 by winning the second-tier Challenger event in Nottingham last month.
Dedicating the win to his children, Cilic added: "They are one of the reasons that I kept such a great passion for this sport.
"Also always running after them keeps me in great shape! I'm so pleased they are here supporting me."