Oakland's Sonny Gray has played a key role for baseball's best pitching staff, while Chris Young's comeback from injuries has helped Seattle exceed expectations.
Both right-handers will try to cap outstanding first halves with a victory Sunday, though Young will have an opportunity to lead the Mariners to their first three-game sweep of the visiting Athletics in almost three years.
Despite losing three of four following a six-game win streak, Oakland (58-36) has the most wins in club history entering the All-Star break and leads MLB with a 3.12 ERA.
Gray (9-3, 2.97 ERA) hasn't lost in almost a month, going 3-0 with a 3.12 ERA in his last four starts - all Oakland victories. He allowed one run over seven innings for the second straight start in Tuesday's 6-1 home win over San Francisco.
The second-year pitcher picked up his 14th career victory spanning just 30 games. Only Tim Hudson (15) compiled more wins in his first 30 appearances with the A's.
"You're just going out there and trying to keep it going," said Gray, who is 3-0 with a 1.44 ERA in four starts versus Seattle.
He beat the Mariners (51-43) twice in April, surrendering one earned run over six innings in a 6-3 home win before allowing one run over seven in a 3-1 road victory.
Kyle Seager is 1 for 10 when facing Gray, but had a home run and a double in Saturday's 6-2 victory at Safeco Field. Robinson Cano extended his recent hot stretch, going 3 for 5 with a three-run homer. He's batting .434 with 13 RBIs in his last 13 games.
Even though the offense broke out with its highest run total since July 1, the Mariners have won the first two in this set behind a pair of outstanding efforts on the mound. Felix Hernandez gave up two runs over eight innings in Friday's 3-2 victory before Hisashi Iwakuma allowed two runs over 8 2-3 Saturday.
The Mariners haven't swept Oakland in at least three games since Aug. 1-3, 2011.
Chris Young (8-5, 3.09) will look to provide another solid performance after going 3-1 with a 1.69 ERA in his last five starts.
The right-hander has been one of Seattle's biggest surprises following six years of shoulder problems. He fell short in his bid to win a fourth straight decision Tuesday, when he allowed two runs over seven innings in a 2-0 home loss to Minnesota.
"He's been great," manager Lloyd McClendon told MLB's official website. "Coming into spring training, I didn't know what to expect. But I couldn't have expected this."
Young has pitched particularly well at Safeco, going 5-2 with a 2.24 ERA in nine starts. He allowed four hits over six innings there versus Oakland on April 13, but did not factor in the decision as the Mariners rallied late for a 3-0 win.
The veteran also has given up two runs and three hits over eight innings in two road appearances - one start - against the A's. He's 3-1 with a 2.52 ERA over seven career meetings.
Brandon Moss is 2 for 6 with a home run when facing Young this season. He's 4 for 23 over his last six games, though he hit a two-run homer in the ninth inning Saturday to spoil Iwakuma's shutout.
Alberto Callaspo won't be available because of a hamstring strain, while Coco Crisp is day to day with a sore neck.