Diaz's save Sunday in the Mariners' 4-3 victory in 10 innings was his 46th of the season, which leads all of Major League Baseball.
"I got a welcomed call from our closer. He said, 'If we got the lead, I'm in the game.' Sometimes you've got to do that. That's kind of how Eddie's wired and how this team is wired," Mariners manager Scott Servais said after the game.
The most recent pitcher to have four saves in a single series was Joe Nathan, who did it for the Minnesota Twins in a series against the Detroit Tigers. John Wetteland (for New York Yankees against the Baltimore Orioles in 1996), Kent Tekulve (for the Pittsburgh Pirates against the Astros in 1978) and Sparky Lyle (for the Boston Red Sox against the Oakland Athletics in 1970) are the only other pitchers to achieve the feat.
The Mariners tied Sunday's game in the ninth inning when Ryon Healy hit a home run with two out off the Astros' Hector Rondon. Mitch Haniger's double off Houston's Roberto Osuna in the 10th scored Dee Gordon from first base with the go-ahead run. Gordon had singled as a pinch hitter with one out in the 10th off Osuna.
The victory pulled the Mariners, in third place in the American League West, within four games of the first-place Astros. Seattle trails the second-place A's by one game for the second AL wild-card spot.
ESPN Stats & Information contributed to this report.