Mark McGwire, 65, 1999
After breaking Roger Maris' longstanding record the previous year, another similarly stratospheric total in 1999 didn't appear to be in the cards for McGwire. He did hit a home run on Opening Day against the Brewers, but he had just seven in St. Louis’ first 30 games. From July 2 to Aug. 5, however, he went yard 21 times in 29 games. He reached 50 on Aug. 22, the Cardinals' 124th game, one fewer than he needed in the previous year. A 19-game span with just four homers from Aug. 25 through Sept. 13 appeared to spell the end of the chase, but after sitting for two games, he closed with 10 homers in his final 15 games, and six in his final seven.