SAN FRANCISCO -- Blake Snell struck out a career-high 15 over six innings as the San Francisco Giants beat the Colorado Rockies 4-1 in a doubleheader opener Saturday.

Snell, the reigning NL Cy Young Award winner, struck out 15 of 18 batters in the no-decision. It was the most strikeouts for a Giants pitcher since Tim Lincecum had 15 on July 27, 2009, against the Pittsburgh Pirates.

Snell's strikeout total was one short of Giants' single-game record, which was set by Christy Mathewson in 1904 and tied by Jason Schmidt in 2006.

Snell said he noticed how many strikeouts he had and was attempting to top his prior career best of 13, accomplished four times.

"I wasn't trying early on," Snell said. "Last couple innings, I was really trying. I knew where I was at: I need to get 13. I want 13. After the sixth, I was well aware of what I was trying to do."

Snell also joined Randy Johnson, Roger Clemens and David Cone as the only pitchers in the expansion era (since 1961) to record 15 strikeouts or more in the first six innings of a game, according to ESPN Stats & Information research.

The left-hander threw 102 pitches, allowing two hits and no runs.

Snell, 31, signed a two-year, $62 million contract as a free agent but is 0-3 in 10 starts, spending time on the injured list with a strained adductor muscle in his left thigh and a strained left groin.

"He's always been kind of a second-half guy," Melvin said. "Last year, when he went on a run like this, not only was it similar to this type of stuff every night that he was either giving up no runs or one run, he was able to pitch out of jams. He was able to get a strikeout with a man on third. It's kind of his forte."

Snell said he's not worried about being dealt ahead of Tuesday's trade deadline as long as the Giants string together some wins. Snell can opt out of his contract after the World Series and become a free agent, giving up a $30 million salary for 2025.

"We just got to win," Snell said. "If we win, I ain't going nowhere. At least I believe that."

The Giants won the second game 5-0 for the doubleheader sweep.

The Associated Press contributed to this report.