It was the first time for both in the dance off and although not a surprise for Susan, Alexandra found herself in danger despite being favourite to win.
Her dance off appearance was the latest in a string of shocks, following Debbie McGee and of course Aston Merrygold.
On Saturday, Alexandra and her pro partner Gorka Marquez danced the Rumba to Halo by Beyoncé to score 32 points from the judges.
Susan and her pro dancer Kevin Clifton had performed the American Smooth to Beyond the Sea by Bobby Darin, scoring 21 marks from the judges.
Alexandra placed third in the leaderboard whereas Susan was bottom for a second week running.
As usual, the results from the public vote were added to the judges' scores to decide the bottom two in the dance off tonight.
First to be announced as in the dance off were Alexandra and Gorka.
Alexandra reacted: "I've loved every second of this journey, it's been an honour to be here so I'm going to give it everything."
Before they danced again, Strictly judge Shirley Baallas advised: "You've got great energy, go with that passion and that feeling you bring every week."
After also being announced as in the dance off, Susan joked: "It's going to be a difficult dance off. I think if we worked on our technique we can make up those points! I've never seen a more uneven contest in my life."
Bruno Tonioli told the pair: "You bring so much fun and joy to the dance, it's a pleasure to watch, just enjoy yourself."
Tonight's Strictly Come Dancing 2017 results
Both Alexandra and Susan danced again for the judges, who had to decide which couple should stay in the competition.
Craig Revel Horwood, Shirley Ballas, Darcey Bussell and Bruno Tonioli all voted to save Alexandra and Gorka.
Therefore, it was Susan and Kevin who were voted off Strictly tonight.
Strictly Come Dancing 2017 is back next Saturday and Sunday night, live on BBC One.