Contador, who claimed the 2007 and 2009 Tour de France victories, and Siddle expressed their support for Porte via Instagram - as the Aussie strives for a podium finish in the general classification.
"My friend, tomorrow you have a great opportunity, FULL GAS!!!" Contador said on Saturday morning (AEST), responding to Porte's original post.
Porte, 35, in turn responded to the Spaniard with "@acontadoroficial full, full gas."
Siddle, who represented the Australian cricket team on 89 occasions before retiring last year, also ensured he supported his countryman via Instagram.
"Bring on tomorrow!" Siddle said, as a comment to Porte's post.
The penultime stage of the three-week grand tour of more than 3,300km and 83 hours of racing will decide the race winner with Primož Roglič (Jumbo-Visma) holding a 57 second advantage over his nearest challenger, fellow Slovenian Tadej Pogačar (UAE Team Emirates).
However, the focus for the man in third on the general classification, Colombia's Miguel Angel Lopez (Astana), is likely to be on defending his place against challenges from Porte (Trek–Segafredo) and Spaniards Mikel Landa (Bahrain-McLaren) and Enric Mas (Movistar).
Porte will have to make up 99 seconds during the 36.2km Stage 20 individual time trial - an uphill trek from Lure to La Planches-des-Belles-Filles - on Saturday night (AEST), if he is to grab a place on the podium.
At his best, the Australian can be competitive with the likes of Roglič. You would expect that he'll need to do that here and then hope that López is not on one of his good days.
The 2020 Tour de France continues with Stage 20's 36.2km individual time trial from Lure to La Planche des Belles Filles, a race against time for Australian Richie Porte to find 99 seconds and the third step on the Paris podium. Watch on SBS and SBS On Demand from 8:30pm (AEST) and the SBS ŠKODA Tour Tracker from 8:50pm (AEST).