Elton John Duets 'Cold Heart' With Dua Lipa in London

Elton John has already said "Goodbye Yellow Brick Road" ... finishing up his farewell tour last year -- but, lucky fans got a chance to see him in London -- at Dua Lipa's show.

The legendary singer-songwriter stopped by Dua's concert at Royal Albert Hall in the UK Thursday ... joining his fashionable colleague onstage as they dueted their popular song "Cold Heart" for the audience.

Check out the clip ... EJ's iconic voice still reverberates around the famous venue as he belts out the hit track inspired by his 1972 song "Rocket Man."

The two shared some laughs onstage as they cut through the song ... holding hands and hitting every note perfectly.

Good to see Elton onstage just a couple weeks after getting real about his health issues. Earlier this month, he talked about all the internal organs he's missing, as well as the numerous areas where he's needed joint replacements.

And, last month, Elton said he got a severe eye infection, which limited his vision in one eye ... so, he's dealing with his own set of challenges every day.