Taylor Swift welcomed Robbie Williams to the stage during her Saturday night concert at London's Wembley Stadium to perform the British singer's 1997 hit "Angels."

Swift opened the ballad alone at the piano before Williams – wearing a Reputation Tour shirt – emerged onstage unannounced, sparking a thunderous response from the London crowd.

"What an honour to sing with you tonight @taylorswift13 I’ve got a proper crush," Williams tweeted after the performance. While never a major hit in the U.S., the multi-platinum "Angels" was voted by the British public as the best song of the past 25 years in 2005.

At Swift's Friday night concert at Wembley, the singer brought out Niall Horan to perform the former One Direction singer's "Slow Hands." Previous Reputation Tour onstage guests include Shawn Mendes, Troye Sivan, Selena Gomez and, at nearly every show, openers Charli XCX and Camila Cabello on "Shake It Up."

The second Wembley Stadium concert concluded Swift's brief six-date, three-stadium European trek, the second leg in the singer's worldwide Reputation Stadium World Tour. Following a week off, Swift's jaunt returns to North America on June 30th with a Lexington, Kentucky gig; the 31-date leg itself ends October 5th and 6th with a two-night stand at Arlington, Texas' AT&T Stadium.