Pitch counts and closers.
Johan Santana had a 2-hit shutout going through 8 innings yesterday — with 17 strikeouts. The record for most strikeouts in a 9-inning game is 20.
Santana threw 112 pitches, so instead of coming back out in the 9th inning with a chance to tie the record, he was replaced by closer Joe Nathan.
Was he tired? Well, he struck out six of the last seven batters he faced, so it sounds like he was just warming up.