Nadal, a 20-time Grand Slam singles champion, advanced without taking the court when Dutch opponent Tallon Griekspoor withdrew.
"We love you Serena," fans screamed as the 23-time Grand Slam title winner appeared for what could have been the final singles match of her career.
The victory sent the 21-year-old from Poland into a Saturday final with Jabeur, who thrashed France's Caroline Garcia 6-1, 6-3 in Thursday's other semi-final.
Jabeur was leading 6-3, 2-1 when Belinda Bencic signalled she couldn't continue in the final after hurting her left ankle in a fall in the first set. "I told her, ‘Forget about it, forget about today.’
After announcing earlier this month that the countdown to her retirement had begun, Williams will step on court at Arthur Ashe Stadium at Flushing Meadows for what could be her final appearance in a Grand Slam singles event.
“I’m really happy with this,” said Raducanu in her acceptance speech. “I’ve watched Sports Personality of the Year growing up and it’s an honour to be among those past winners.
Peng, the former doubles world number one, is absent from Melbourne and there are fears for her wellbeing after she alleged online in November that she had been "forced" into sex by a Chinese former vice-premier during a years-long on-and-off relationship.
The seventh seed overcame the French 17th seed in their quarter-final 6-4, 6-4, 3-6, 3-6, 6-2 to become the first Italian man to reach the last four of the Australian Open.
The 16-year-old arrived at the tournament with a world ranking of 279 but she dominated the two-time Australian Open winner, racing into a 6-2, 3-0 lead before the veteran Belarusian quit.
Under the lights at Rod Laver Arena, the legendary Spaniard demonstrated his trademark big-match mentality, overcoming the Russian Medvedev 2-6, 6-7(5), 6-4, 6-4, 7-5 in an epic five hours and 24 minutes final to win the trophy.