In a May 22 matchup against the St. Louis Cardinals, Robertson allowed one run and four hits in a 5–2 victory; after the game, catcher Gregg Zaun said that it was "absolutely the best he's thrown." After the win against the Cardinals, Robertson won his next two starts, then he had three more matchups with a no-decision. Robertson continued to win games over the following months, and on July 22, he won a match against the Pittsburgh Pirates, 2–0. In the game, Robertson allowed three hits in what was his ninth victory in a row without a loss, as well as his sixth victory in his past six starts for Houston. His streak ended on July 27, as he only lasted innings in a 5–3 loss to the Chicago Cubs in what was at the time the shortest start of his major league career. A month later, Robertson broke the team record for most victories by a left-handed rookie pitcher when he notched his 13th win against the Los Angeles Dodgers on September 2. His last start of the season on September 26, however, was his worst; he only pitched a third of an inning, allowed three earned runs, and threw 7 of 18 pitches for strikes as the Milwaukee Brewers won, 12–5. Robertson finished the 2003 season with 15 victories, 9 losses, a 5.10 ERA, and 99 strikeouts. He also finished in seventh place for the Rookie of the Year Award.