"I'm throwing about 85 percent," the Oakland pitcher said.
Oakland pitchers have allowed only four runs in winning their last five games.
Welch has won four decisions in a row and is the sixth Oakland pitcher ever to win 20 games.
It was the 14th time in 18 games that Oakland pitchers had allowed no more than two runs.
Welch was seeking to become the fourth Oakland pitcher to win 18 games this season.
Clement said the Oakland pitchers served as a good model for the Marlins' corps.
Gene Nelson (2-2), the third of four Oakland pitchers, allowed one hit in three innings.
Stewart became the first Oakland pitcher to win seven straight games at the start of a season.
It was the Yankees' fortune that the Oakland pitchers were worse.
The Oakland pitchers were trying to complete the seventh combined no-hitter in the major leagues.