user8291: As a Magnus Carlsen fan, it was a bit disappointing for me to see him finish clear second in Biel 2018, one and half points after Mamedyarov, after starting the tournament with two wins - and it could have been two points - had not the much lower-rated Georgiadis blundered in a drawn endgame. Although not a bad performance per se, I was wondering what were the worst tournament performances of reigning World Champions? 

user4362: For Carlsen that was probably Norway Chess 2017, where he finished second to last on -1.

user9025: Looking through this list of strong tournaments, among those that stand out are:

AVRO 1938, where Alekhine finished 6/8.
Tillburg, 1996, where Karpov finished 6/12.
1st Grand Slam Masters Final (2008), where Anand finished in last (6th) place.


8291: <p>As a Magnus Carlsen fan, it was a bit disappointing for me to see him finish clear second in Biel 2018, one and half points after Mamedyarov, after starting the tournament with two wins - and it could have been two points - had not the much lower-rated Georgiadis blundered in a drawn endgame. Although not a bad performance <em>per se</em>, I was wondering what were the worst tournament performances of reigning World Champions? </p>
 4362: <p>For Carlsen that was probably <a href="http://www.chessgames.com/perl/chess.pl?tid=90303" rel="noreferrer">Norway Chess 2017</a>, where he finished second to last on -1.</p>
 9025: <p>Looking through this list of strong <a href="https://en.wikipedia.org/wiki/List_of_strong_chess_tournaments" rel="noreferrer">tournaments</a>, among those that stand out are:</p>

<ul>
<li><a href="https://en.wikipedia.org/wiki/AVRO_1938_chess_tournament" rel="noreferrer">AVRO 1938</a>, where Alekhine finished 6/8.</li>
<li><a href="https://en.wikipedia.org/wiki/Tilburg_chess_tournament#1996" rel="noreferrer">Tillburg, 1996</a>, where Karpov finished 6/12.</li>
<li><a href="https://en.wikipedia.org/wiki/Bilbao_Chess_Masters_Final" rel="noreferrer">1st Grand Slam Masters Final (2008)</a>, where Anand finished in last (6th) place.</li>
</ul>
