He made his senior international debut in 2011, and was part of Germany's squads which reached the semi-finals of UEFA Euro 2012 and won the 2014 FIFA World Cup.

He was also named as an unused substitute in 11 reserve matches, including the final of the Manchester Senior Cup, earning him a winners medal.

[5] The following season, he shared Under-18 goalkeeping duties with Ben Amos, but also made his debut for the reserves on 15 March 2007, a 3–0 home defeat to Sheffield United.

[6] Despite that inauspicious start, he made amends four days later by keeping a clean sheet in the 2–0 Manchester Senior Cup win over Oldham Athletic.

[7] The 2007–08 season marked Zieler's emergence as a full member of the reserve team, as he and Tom Heaton shared the majority of the goalkeeping duties, each making 11 appearances.

In May 2008, Zieler won another Manchester Senior Cup winners medal, as he played the full 90 minutes and kept a clean sheet against Bolton Wanderers.

[14] Zieler made his debut for Northampton Town on 21 February 2009, playing for the full 90 minutes of the Cobblers' 2–0 home defeat by Walsall.

[17] Upon returning to Manchester United, Zieler recommenced his duties as reserve team goalkeeper, conceding just one goal in his first four games back.

[18] He returned to action in the 2009–10 pre-season, but struggled to regain his position in the reserve team; his appearances throughout the season were sporadic and he was most often named as an unused substitute.

[30] In both encounters against Bayern Munich in the league, Zieler conceded eleven goals against the club, as Hannover lost 5–0[31] and 6–1 on 24 November 2012 and 20 April 2013 respectively.

[34] Zieler then made his 100th league appearance for the club on 21 December 2013 against Freiburg, but was booked in the 88th minute after fouling Admir Mehmedi, resulting in him being substituted and Hannover losing 2–1.

Ahead of the 2015–16 season, newly appointed manager Michael Frontzeck expected Zieler to be the new captain following the departure of Lars Stindl.

[49] He faced more shots on target than any other goalkeeper in the 2015–16 Bundesliga season, making a total of 148 saves, second behind Oliver Baumann, with 90 of them coming from inside the penalty area.

[50] On 3 June 2016, Zieler returned to England for the first time in six years, signing for champions Leicester City on a four-year deal for an undisclosed fee.

[53] Zieler made his Leicester debut on 27 August 2016, replacing the injured Kasper Schmeichel in the 57th minute of the club's 2–1 home win against Swansea City.

Given that the team had another reserve goalkeeper in Ben Hamer, the club chose to sell Zieler to VfB Stuttgart for an undisclosed fee after just one season.

During his second season, Stuttgart experienced a severe slump, which resulted in a 16th-place finish and the club's eventual relegation, after losing to Union Berlin in the 2018–19 play-offs.

In early August 2020, club chairman Martin Kind stirred controversy by publicly belittling Zieler and encouraging the player to seek a new employer.

[66] On 15 August 2012, he started in net in a friendly match against Argentina and was sent off, becoming the first German goalkeeper to earn a red card since the founding of the Germany national team.

[67] On 2 June 2014, Zieler was again named the third goalkeeper of the German squad for the 2014 FIFA World Cup, remaining as back-up as they reached the final and won it for the fourth time.