Both advanced over Chinese players on Thursday to set up a championship match involving two former Australian champions.
Top-ranked Williams wasted four match points before finishing off a 7-6 (4), 7-6 (1) semifinal win with an ace against Li Na on Thursday, a day after her sister Venus lost to the Chinese player in the quarterfinals.
Henin overwhelmed 2008 Wimbledon semifinalist Zheng Jie 6-1, 6-0 in 51 minutes, the shortest match of this tournament and the most lopsided semifinal at the Australian Open since Chris Evert beat Andrea Jaeger by the same score in 1982.
Williams leads Henin 7-6 in career head-to-heads, although they've never met in a Grand Slam final. Williams won their last match, at Miami in 2008, just before Henin retired suddenly while holding the No. 1 ranking.
"I can't wait for the final. It's such an amazing chance that I have to play another final in Melbourne," said Henin, who won the 2004 title in Melbourne and retired with stomach problems during the 2006 final against Amelie Mauresmo. "It's a very special occasion, but the dream continues.
"I'm going to play the No. 1 player in the world in a Grand Slam final. She's a real fighter."
Serena Williams has a 100 percent conversion rate so far in finals at Melbourne Park, winning the title every time she's played for the championship since beating Venus here in 2003. The winning sequence has been every odd-numbered year so far.
"I really should have won sooner ... I had so many match points and I blew it and I knew I couldn't mess up my serve because she never gives up," Williams said after posting her 50th career win at Melbourne Park. "She's a real, real amazing fighter.
"Every time I had match points she came up with some big serves and great shots. She just goes for broke."
Williams, the 11-time Grand Slam singles champion, was joining Venus in a doubles semifinal in the afternoon.
China had two players into the semifinals of the same major for the first time but still is yet to get a finalist.
Henin has won seven Grand Slam singles titles but is into her first major since the 2008 Australian Open, when she lost in the quarterfinals to eventual champion Maria Sharapova.
Henin is unranked and two tournaments into a comeback from 20 months off the tour, hoping to emulate fellow Belgian Kim Clijsters' win at the U.S. Open.
Clijsters was only three tournaments into a comeback from two years off, and playing on a wild card entry, when she beat both Williams sisters en route to winning the title at New York.
Williams is hoping for better against a Belgian on the comeback this time. Her semifinal loss to Clijsters in New York cost her a record $82,500 fine for a profanity laced tirade against a line judge who called her for a foot fault.
Against Li, she got on top by breaking in the opening game.
Li fended off a set point in the ninth game before holding, then broke Williams' serve in the 10th game to even the match at 5-5.
In the tiebreaker, Williams picked up four of her last five points on unforced errors by Li and then clinched the 58-minute set with a second-service ace.
The second set went with serve, with Li fending off three match points in the 10th game and another in the 12th to force a second tiebreaker. Again, Williams dominated the tiebreaker to race to a 6-1 lead, closing with her 12th ace of the match.
Venus Williams, who had a chance to serve for the quarterfinal on Wednesday against Li before she lost, was watching from the stands. Chinese fans waving their red national flags were sitting in the rows behind her.
No. 5 Andy Murray and No. 14 Marin Cilic will meet in a semifinal Thursday night. Murray ousted defending champion Rafael Nadal and Cilic beat U.S. Open champion Juan Martin del Potro in the quarterfinals.
With Jo-Wilfried Tsonga taking out No. 3 Novak Djokovic late Wednesday night, there was nobody left in the draw who has beaten Roger Federer in a major. Federer plays Tsonga in a Friday night semifinal.