[10][11] On 4 August 2015, he greatly contributed to 3–2 away win with a strong performance in the third qualifying round of the 2015–16 UEFA Champions League against Ajax.

[14] Rapid in October stated their intention to extend Petsos' contract after he had been one of the team's best players during its strong start to the season.

[15] On 18 October 2015 he returned to the team, starting in an Austrian championship game against Wolfsberger AC, but suffered an injury in the first quarter of the match.

[17] In January 2016, he announced he would not accept Rapid's offer to renew his contract which was set to expire in summer 2016 and would instead leave the Austrian club after three years.

"With the commitment of Thomas Delaney and the return of Florian Grillitsch, the competition for Thanos has become even bigger," said Managing Director Frank Baumann.

[27] In January 2020, after being out of contract for half a year, Petsos joined WSG Swarovski Tirol, last in the Austrian Bundesliga table.

On 5 August 2011, he was called up to the senior team for the first time by manager Fernando Santos for the friendly match against Bosnia and Herzegovina, and three days later he made his international debut.

After almost four years, on 3 September 2015, he was called up from manager Kostas Tsanas for the Euro 2016 qualifier matches against Romania and Finland, but was injured in a training session of the national team.

The Düsseldorf-born player feels more secured in the field on the position of the central defensive midfielder, but in Germany he played sometimes even as a right-back or even a centre-back.