Polona Hercog started playing tennis aged four. Initially trained in Slovenia, but moved to Italy at aged 14 and trained there before turning pro. In 2006 she made her qualifying debut in Portoroz, where she played her first WTA main draw match in 2007.
2009 was her first Top 100 season. She made Top 100 debut on July 6 (rose from No.113 to No.89), won four singles titles on ITF Circuit and reached the quarter-finals in Fès; reached 2r four times (incl. Roland Garros); fell 1r three times (incl. US Open) and in qualifying three times (incl. Wimbledon);
In 2010 she made her Top 50 debut on June 7 and finished the season as a Top 50 for the first time after being runner-up in Acapulco (l. to V.Williams in final) and reaching the semifinals in Portoroz (l. to Chakvetadze) and four quarterfinals in Budapest, Prague, Copenhagen and Luxembourg. That year she won her only two doubles titles in Acapulco (with Strycova) and Seoul (partnering Goerges).
2011 was her breakthrough season, highlighted by her maiden singles title in Bastad, where she denied home favourite Johanna Larsson to claim her first WTA title. Larsson achieved her career‐high ranking of No.35 in September 2011 and posted her first Top 40 season finish at No.36.
Her ranking dropped in 2012 as she struggled with a back injury (she had to retire in Brisbane and withdrew from Rome, Brussels, Bad Gastein, 's-Hertogenbosch and Tashkent). She managed to defend her title at the Swedish Open as she beat Mathilde Johansson
In 2013 she played one match in first four and a half months at Australian Open. She suffered a wrist injury and was forced to withdraw from Auckland, Bogotá, Acapulco, Indian Wells, Charleston, Katowice and Marrakech). She returned in May at No.133, reached the quarter-finals in Nürnberg, Québec City and Osaka and was ranked No.66 by November 4 year-end rankings.
Also in 2014 she retired at Australian Open, with right shoulder injury, and came back on court in April. Her best results diring the rest of season were three quarter-finals at Marrakech, Oeiras and Bucharest
Hercog ended 2015 ranked No.71, for her seventh Top 100 season. The best result during the year was a smifinal run in Bucharest (l. Schmiedlova) and a quarter-final finish in Bad Gastein (l. Knapp).
The Swede suffered three injuries following 2016 US Open (l. Kerber in via ret.) including a scapula stress fracture, left knee injury and right wrist injury. She completed the year at No.139 for her her first non‐Top 100 season since 2008 (finished No.243). Her 2016 highlights included a runner‐up finish at WTA $125k Series event in Bol (l. Minella) and the quarter-final run in Bucharest.
In 2017 she missed the first five months of the season as she recovered from the past year's injuries. She clinched the third-round at Wimbledon and won six singles title on ITF Circuit.