Nathan spent all of 2003 with the Giants in the bullpen after marrying Lisa Lemoncelli, his girlfriend of five years, in November 2002. This was a breakout year for Nathan, starting the season with 23 scoreless innings en route to a 12–4 record in his first full year as a reliever. His 78 appearances put him high on the list of most-used pitchers for the season as one of the best setup men in the NL, allowing no runs in 15 appearances from July 18 to August 20. His 12 wins in relief led the majors. The Giants won the National League West by 15 games and drew the Florida Marlins, the National League's wild card winner, in the NLDS. Nathan was hit hard in that series, blowing his only save opportunity. His team fared no better, winning Game 1 behind Jason Schmidt's complete game shutout before dropping the next three.