Raducanu arrived in Eastbourne as the British number one, having enjoyed a run to the quarter-finals at Queen’s a fortnight ago.
But as she prepares for her fourth Wimbledon appearance – where she has twice reached the fourth round – her performance at Eastbourne looked laboured.
Some impressive serving in the opening set gave her the lead but the wheels seemed to come off in the second, and a commanding performance from Joint levelled the match.
World number 38 Raducanu pulled out of the Berlin Open last week because of a back issue but, while she seemed uncomfortable at times, she did not call for the trainer at any point against Joint.
Raducanu held back tears after securing victory on Tuesday and said she was “mentally not really present” at points during the match.
She slumped in her chair at the change of ends at times on Wednesday, shaking her head in frustration – but she dialled back in to produce a thrilling end to the third set.
After breaking Raducanu in the first game of the decider, Joint saved break-back points at 3-2 then broke for a second time after Raducanu followed a double fault with a long forehand.
But Raducanu rediscovered some momentum, twice breaking as Joint served for the match – and she even seemed to surprise herself with a remarkable break to love for 5-5.
The to-ing and fro-ing continued, both players exchanging breaks before Raducanu denied Joint for a third time as she served for the match and forcing a tie-break.
But an unfortunate net cord paved the way for Joint, who took the lead and sealed her place in the quarter-finals with an ace.
Joint, who was ranked 684th in the world at the beginning of 2024, will face world number 69 Anna Blinkova in the last eight.
She will also make her Grand Slam debut at Wimbledon when it begins on 30 June.