Taylor Swift reveals the perfect moment for Travis Kelce to join her on stage, and he succeeded after just one rehearsal... hinting at more surprise appearances to come.

Taylor Swift is looking back on her boyfriend Travis Kelce's Eras Tour debut during night 3 at Wembley Stadium.

Taylor Swift is feeling all the feels after her beau, Travis Kelce, joined her onstage Sunday during the Eras Tour.

On Monday, the "Bad Blood" singer took to Instagram to reflect on her three-show run at London's Wembley Stadium, which began on Friday. One of the biggest highlights came on night three when the Kansas City Chiefs tight end surprised the crowd when he joined her onstage. 

"Wembley Stadium round one was so surreal. Looking out into crowds of 90,000 fans each night and hearing that VOLUME… Those were some of the loudest crowds I’ve ever heard," the 34-year-old singer wrote. "Acoustic sections went OFF. I got to sing 'Castles Crumbling' with @yelyahwilliams for the first time - hadn't performed with her since 2011 and man my heart was just so full of love for her and our friendship. Then the impossibly gifted and wonderful @gracieabrams came and surprised the crowd with the first performance of our song 'us.'"

Taylor Swift took to Instagram to swoon over her boyfriend, Travis Kelce's, surprise Eras Tour performance. - Gareth Cattermole/TAS24/Getty Images for TAS Rights Management

Swift ended her post by reacting to her boyfriend's big Eras Tour debut. 

"And I'm still cracking up/swooning over @killatrav's Eras Tour debut 🥰 Never going to forget these shows. Can't wait to do it 5 more times in August. Up next: Dublin!! 🇮🇪 📷: @kevinmazur / @garethcattermole / @hogieaaa__ / @gettyimages," she wrote.

The "Bad Blood" singer's post began with a picture of Kelce -- dressed in a tux and top hat -- onstage with her and other dancers during the skit ahead of Swift's "I Can Do It With a Broken Heart." It also included photos of her and Hayley Williams and Gracie Abrams performing their separate duets. 

Swift rounded out the post with a video of Kelce's big moment. 

Taylor Swift was joined onstage by Travis Kelce during night 3 of her Wembley stadium shows. - Gareth Cattermole/TAS24/Getty Images for TAS Rights Management

A source tells ET that Swift was waiting for the right moment for her boyfriend the join her onstage, and he nailed it after just one night of rehearsal.

"Taylor and Travis have recently discussed Travis making an appearance onstage at the Eras Tour, when timing was right, and they finally made it happen," the source says. "Travis was so excited to support Taylor onstage, and Taylor couldn't be happier to feature him, especially during such a fun, pivotal scene in the show. Travis only had one night of rehearsal, the evening before he performed onstage, and didn't miss a beat. Taylor was beaming from ear to ear as Travis took on the dapper role. The two are hoping he can make a cameo again in the future."

The Super Bowl champion -- who was spotted in the VIP section -- surprised fans when he took the stage. For his bit, Kelce carried his girlfriend in his arms before the other dancers stripped her of her dress and revealed a white two-piece set underneath, all as Kelce watched from afar, fanning himself and smiling from ear to ear. 

Before running off the stage, No. 87 adorably pretended to apply makeup to his lover's face, grinning as Swift -- acting like a rag doll of sorts -- pretended to be over the whole situation. 

After the moment, Kelce was greeted by Swift's other notable support system, her mother Andrea Swift, and friends Sophie Turner and Gracie Abrams. Kelce, along with his brother, Jason Kelce, and sister-in-law, Kylie Kelce, were on hand for all three performances. 

Swift and Kelce kept the party going -- until four in the morning -- after the show as they hit up hot spot Chiltern Firehouse and partied late into the night with Turner and Ellie Goulding, who were also at the show.

Taylor Swift did three back to back nights at Wembley Stadium in London. - Kevin Mazur/Getty Images for TAS Rights Management

The couple was spotted holding hands outside of the venue and sporting coordinating outfits. Swift turned heads in a colorful crochet mini-dress featuring long, slightly flared sleeves. She accessorized with yellow open-toe chunky heeled sandals and a brown shoulder bag. As for Kelce, he embraced a casual chic style, donning a white shirt paired with matching pants and a blue bucket hat. 

Swift's IG post comes after she and Kelce went IG official on Friday. Following her first show at Wembley, the "Lavender Haze" singer took to Instagram to share a selfie featuring her, Kelce, Prince William, Princess George and Princess Charlotte -- who attended the concert on the future king's birthday.

"Bday M8! London shows are off to a splendid start 🇬🇧🇺🇸🤝 @princeandprincessofwales," she captioned the photo. 

Swift's weekend in London was a star-studded affair as her three shows brought out some of the biggest celebrities including Hugh Grant, Salma Hayek, Ashton Kutcher, Mila Kunis, Tom Cruise and more. 

In May, a source told ET that Kelce and Swift -- who began their love story last year -- have family and friends suspicious of an engagement. 

"Taylor and Travis are doing amazing, and their loved ones see an engagement coming sooner than later," the source said. "They make a great match and there's no question about that."