Note: with three exceptions, Viktor Korchnoi faced in the Candidate matches always a younger player (the exceptions are Petrosian by a nose, Geller, and of course Reshevsky). Karpov is
twenty years younger than Korchnoi, Kasparov thirty-two years. Ponomariov (drawn friendly match in 2001) even fifty-two years!
Play-off matches to decide a tournament, from Korchnoi vs. Kotkov (play-off
after Poltava 1956) to Korchnoi vs. Gallagher (Swiss national championship 2011) are NOT counted for obvious reasons, they are (only) used as tie-breaker, and if won, listed under the corresponding tournament event.
Knock-out encounters within a closed competition (in the mid and late 1990s some tournaments switched to a k.-o. format as in tennis for one or the other edition, and the FIDE World Chess Championships were decided this way) are naturally
NOT counted even if there was a considerable number of games (eg. Korchnoi vs. Dolmatov 4.5:3.5 after extra-time rapids, 2nd stage at Las Vegas FIDE WCC 1999).
The Candidates matches vs. Hjartarson in 1988
(lost) and vs. Sax in 1991 (won) were in fact rather 1/7-finals than 1/8-finals, because in both cycles Anatoly Karpov was directly seeded in the quarterfinal, subsequently there were seven (and not eight) matches in the prior round.