ST. LOUIS -- As soon as Cardinals relief pitcher Ryan Helsley let go of the strike-three fastball that blew D-backs second baseman Ketel Marte away during the eighth inning on Sunday afternoon, he knew there was something different about it.

Just to make sure, he took a quick peek at the Busch Stadium radar reading for confirmation: 103.1 mph.

“When I struck him out and I was walking around, that one felt a little harder and came out a little better,” Helsley said after the Cardinals’ 7-5 victory. “I just looked up, and I was like, ‘Wow!’ It kind of caught me by surprise.”

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Helsley’s 103.1 mph four-seamer was the fastest MLB pitch this season. Previously, Mariners reliever Andrés Muñoz had thrown the fastest pitch of the season at 102.8 mph.

It was the fastest pitch the 27-year-old Helsley said he had ever thrown in any level of baseball. Previously, the right-hander’s pitches had been clocked at 101.5 mph on three different occasions at the MLB level. On Sunday -- when Helsley pitched two perfect innings to record the third save of his career -- he topped his previous fastest pitch five times.

“I haven’t thrown in a couple of days, so I was feeling pretty good,” said Helsley, who threw eight of 18 pitches on Sunday at least 100 mph. “I just wanted to challenge [Marte] with my best pitch, and it just came out a little harder than it had before.”

Helsley’s electric stuff has been evident all season. In 8 1/3 innings over seven outings, Helsley hasn’t given up a run or a walk while striking out 16 hitters. The opposition is hitting .038 off him.

Helsley said his pitching improvement -- and the increased velocity on his fastball -- can be traced to the health of his left knee after he underwent arthroscopic surgery during the offseason. Helsley said he originally injured his knee prior to Spring Training in 2020 and pitched in pain much of the past two seasons before shutting things down near the end of last season.

“It just feels good to be healthy, to be able to trust my stuff and trust that my body is going to be strong again,” he said.

Helsley’s 103.1 mph fastball topped teammate Jordan Hicks’ 102.1 mph pitch on April 17 as the fastest Cardinals pitch this year. Hicks, who tied Aroldis Chapman for the fastest pitch ever recorded at 105.1 mph in 2018, said he was passing the torch to Helsley now that he’s made the transition from reliever to starter for the Cardinals. However, Helsley said he likely wouldn’t hold the fastest pitch record for long.

“I wouldn’t put it past him,” Helsley said of Hicks, who started Sunday’s game and threw three pitches of at least 100 mph. “He’s pretty competitive, so you might see him reach back here in the future.”