Petra Kvitova out in Sydney semis
Kvitova, the 2011 Wimbledon champion, was broken twice in the first set and four times in the second against Pironkova, who finished last season ranked No. 108.
"First time into a WTA final -- it's an amazing feeling," said Pironkova, 26. "I've been waiting for this for a long time."
With qualifiers and the main draw, she now has won seven consecutive matches in Sydney and is aiming for No. 8 in the final, where she'll meet either fifth-seeded Angelique Kerber or American Madison Keys.
Del Potro, the 2009 US Open champion, who finished last season ranked No. 5, relied on his strong serve, with 14 aces to three, and saved seven of 10 break points.
John Isner reached the semifinals of the ATP's Heineken Open in Auckland, New Zealand, after a 6-7 (4), 7-6 (4), 7-6 (5) win over fifth-seeded Philipp Kohlschreiber in a match between former champions.
The quarterfinal match between third-seeded Isner, the 2010 Auckland champion, and No. 22-ranked Kohlschreiber, the 2008 winner and beaten finalist last year, lasted 2 hours, 24 minutes and contained no breaks of serve.
Playing on an injured ankle, Isner finally clinched the match with a vital point against serve in the third-set tiebreak.
"I played against a gutsy opponent so I knew I had to give everything I had," Isner said. "It wasn't that pretty for either of us, and a lot of times, when I'm playing guys as good as him, the match comes down to a point here or there. I got a little bit lucky today."
Isner served 18 aces, put 83 percent of first serves in play and won 82 percent of first-serve points.
In the WTA event at Hobart, top-seeded Sam Stosur reached the semifinals with a 6-3, 6-2 win over No. 34-ranked Bojana Jovanovski of Serbia.
In the exhibition tournament at Kooyong, the former Australian Open venue in Melbourne, No. 7-ranked Tomas Berdych advanced to the final with a 6-4, 6-1 win over Kei Nishikori of Japan. Berdych, who opened the tournament with a straight-sets win over Fernando Verdasco, will next play the winner of Friday's all-French semifinal between Richard Gasquet and Gilles Simon.
No. 8-ranked Stanislas Wawrinka withdrew from the eight-man invitational tournament earlier Thursday, citing fatigue following his title win at Chennai last week. He was replaced in the draw by Russia's Mikhail Youzhny.