Seattle Mariners - for one, promising day, anyway.

By Associated PressSEATTLE (AP) - Younger and slimmer was certainly better for the Seattle Mariners - for one, promising day, anyway.

Felix Hernandez, the 20-year-old would-be phenom who has lost 20 pounds since a disappointing 2006 season, thoroughly dominated the defending AL West champion Oakland Athletics in a 4-0 victory Monday. The youngest opening day pitcher in the major leagues in 22 years allowed three hits and struck out a career-high 12 in eight innings.

Seattle had lost 17 of 19 games - including 15 in a row - to the A's while finishing last in the division for the third consecutive year last season.

Richie Sexson's three-run homer off Dan Haren in the sixth inning, after a pivotal error by shortstop Bobby Crosby on a potential inning-ending double-play grounder, provided far more offense than Hernandez needed.

The youngest opening day starter since Dwight Gooden in 1985 threw 97 mph fastballs and often replaced his usual, slow, gunslinger walk off the mound at the end of innings with rousing screams, fist pumps and glove slaps. Only Randy Johnson has had more strikeouts on opening day for Seattle, 14 to begin 1993 and '96.

Hernandez (1-0) threw first-pitch strikes to 21 of the 29 batters he faced, after having control problems throughout his 12-14 season last year. That was as many losses as he had in his first three professional seasons.

Travis Buck, a 23-year-old rookie from Richland, Wash., who had 20 family members screaming for him from a few rows behind home plate, led off the sixth with a double that thudded off the bottom of the center-field wall on a fly. Mark Ellis' sacrifice bunt moved Buck to third. But Hernandez then struck out Jason Kendall for the third time on a 97-mph fastball high and inside before Shannon Stewart flailed at an 0-2 split-fingered pitch that bounced in front of the plate. As the sold-out crowd roared, Hernandez yelled. He then pumped his first three times, slapped his glove and then wildly slapped the hand of catcher Kenji Johjima.

The Mariners' decisive bottom the sixth immediately followed that pick-me-up.

And when Oakland had two on and one out following Eric Chavez's single in the seventh, Hernandez struck out Nick Swisher on another fastball at 97 and then shook off Johjima to settle on a 96-mph fastball that froze Crosby into watching another third strike. Hernandez yelled again and thrust both arms into his sides after his 11th strikeout.

Sexson lifted a low 1-1 pitch from Haren (0-1) into the shrubs just above the top of the center-field wall, after Raul Ibanez's sacrifice fly scored Ichiro Suzuki with the game's first run. Notes: Sexson was 1-for-17 against Haren before his homer. ... Haren allowed four runs and four hits in six innings of his first career opening day start. He walked one and struck out two. ... Oakland DH Mike Piazza went 0-for-4 in his A's debut.