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).
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.
Video of the match Korchnoi vs. Polugaevsky in Évian, France in 1977: https://www.youtube.com/watch?v=JAQH_YD-ZSQ