Sofia welcomes eight players every year - the six highest-ranked International title winners and two wildcards who aren't competing at the BNP Paribas WTA Finals Singapore presented by SC Global.
Though she didn't win an International title this year, Pennetta won something bigger - the Premier Mandatory-level BNP Paribas Open in Indian Wells, the 10th and biggest WTA title of her career.
The Swiss-Italian pair still has a shot at qualifying for the WTA Finals in Singapore.
"My back injury has been bothering me and it's not getting better quickly enough, so unfortunately my season has to come to an end after Tianjin," Jankovic said in a statement on the weekend.
"I was looking forward to seeing my fans in Moscow and Sofia and playing both tournaments, but due to medical advice I must start my rehabilitation now to allow myself to properly heal for next year.
"Thank you all for your support and I look forward to seeing you next year."