The two music legends recorded the song back in 1981 for Motown as the theme song for Franco Zeffirelli's film of the same name and it went on to become a global hit.
During his sold-out concert at the historic venue - which is King Henry VIII's Tudor Palace - Lionel addressed the crowd to tell them that, much to his chagrin, he has never been able to convince Diana to get on stage with him for a rendition of the track and asked the women in the audience to take the soul star's place on the duet.
He said: ''One of my favourite songs to sing is 'Endless Love', and for 37 years I have asked Miss Diana Ross to sing it with me, and for 37 years she has said 'no'. But I still ask her every time. Is she here tonight? No.
''So what I want is all the ladies here to be Diana Ross, will you sing it with me?''
The ladies gleefully obeyed Lionel's wish and after he ended the romantic ballad, he thanked his female fans by saying: ''Who needs Diana Ross? Give yourself a round of applause, we have thousands of singers like Diana Ross in the crowd.''
Lionel, 68, delivered an incredible set made up of his greatest hits, with highlights including solo songs 'Three Times A Lady', 'Dancing on the Ceiling', 'Say You, Say Me' and 'Hello' and Commodores classics 'Brick House', 'Easy' and 'Lady (You Bring Me Up)'.
The R&B icon also paid tribute to his late friend Michael Jackson and other fallen pop peers as he sang 185 charity single 'We Are The World'.
He said: ''Wow, these gigs are the best. I look out and see someone laughing during the saddest song, and someone crying at the happiest song, and I realise that these songs are memories to people. You have lived these songs.
''In my career I've always had competition to strive me on. Me and the Commodores would sit and think, 'How can we beat this guy, or this group,' and it pushed us on. When I think about my memories and I think about my fraternity brothers and sorority sisters, I think of all the people who are no longer with us. Artists like Maurice White, Bb King, Whitney Houston, George Michael, Prince. We will never be able to see these crazy artists perform again but the memories will live on forever. One artist I was lucky enough to write a song with was Michael Jackson, and I want you to sing along and remember him now.''
He finished his show with fan favourite 'All Night Long (All Night)' which got all the audience off their feet and dancing, as they had been for much of the evening.
Lionel plays his third and final sold-out concert at the historic venue on Saturday (09.06.18) but the Hampton Court Palace Festival - which is now in its 26th year - continues for the rest of the month.
Go to Hamptoncourtpalacefestival.com for tickets and information.
Hampton Court Palace Festival remaining shows:
Friday 8 June Paloma Faith
Saturday 9 June Lionel Richie
Tuesday 12 June Joe Bonamassa
Thursday 14 June Jools Holland
Friday 15 June The Beach Boys
Saturday 16 June The Beach Boys
Tuesday 19 June Gary Barlow
Wednesday 20 June Gary Barlow
Thursday 21 June Tom Jones
Friday 22 June Tom Jones
Saturday 23 June Fanfare & Fireworks with the Royal Philharmonic Concert Orchestra
What's new in the music world this week?
LCD Soundsystem was released on this day (January 24) in 2005.
Watch three incredible live performances from Discovr.TV.
Don't miss the K-pop titans' return to Europe.
Listen to Alex Bayly performing 'Animal'.
Listen to 'Can't Get You Out of My Mind' now.
Two weeks ahead of Independent Venue Week, Dry Cleaning made 'Britain's Best Small Venue 2015' (NME) the second port of call on their 2020 tour.
For their last gig of the year, The Libertines came back to their adopted hometown of Margate to finish off their latest tour.