Kylie, who was headlining Radio 2 Live in Hyde Park, teased the audience by starting the duet on her own, with a gospel choir singing Donovan's lines.
After the second chorus she sighed, "I need a dance partner," at which point Donovan emerged from the wings.
He took Kylie in his arms, spinning her around the stage before joining her on the mic for the song's finale.
"He didn't know that was going to happen," laughed Kylie as the song ended.
It later transpired her former Neighbours co-star had cycled to the venue simply to watch the show, before he was swept onstage.
But that wasn't the only surprise Kylie had in store for her fans.
A couple of songs later, she called out Rick Astley and duetted with him on a version of his 1987 debut, Never Gonna Give You Up.
He'd previously serenaded the star with the song at her 50th birthday party this May.
"You are a beautiful, strong, wonderful woman," said Astley as the song ended.
"You've just made my night. You've made my year!"
Kylie responded by singing the refrain of Gabrielle's Dreams - "Dreams can come true".
Both of the stars emerged from - then outgrew - the Stock, Aitken, Waterman hit factory in the 80s.
At one point it was even rumoured that Kylie's hits were produced by speeding up the sound of her label-mate's voice.
She's disproved the theory several times since.
A perfect opportunity arose two songs into her set on Sunday night, when technical snafu forced her to fill time in front of 50,000 people.
"Since we're having an issue: Any requests?" she asked the audience, before delivering impromptu a capella versions of Got To Be Certain, Step Back In Time and Je Ne Sais Pas Pourquoi.
"I know I've got enough hits to keep filling," she laughed, finally getting back on track with a mellow version of In Your Eyes, just one of her 34 Top 10 singles.
The show also included a lively flamenco take on Can't Get You Out Of My Head, incorporating elements of Fleetwood Mac's The Chain; and an acoustic version of her UK debut single, I Should Be So Lucky.
It was a set that illustrated how comfortable Kylie has become with her storied musical legacy - maturing with her audience while embracing and updating the cheesier moments of her back catalogue. Madonna should take note.
The Australian star topped a bill that included Lenny Kravitz, Rita Ora, Manic Street Preachers, Boyzone and All Saints.
All of the performances are available on the Radio 2 website; while Kylie's set was also broadcast on BBC One on Sunday night.
Later this month, she'll be announcing trains as they arrive at stations around the UK as part of a stunt for BBC Music Day.
The star also launches her 15th tour, the Golden Tour, in Newcastle on 18 September.