With the bases loaded in the first inning, Upton got ahead in the count, 2-0, against Los Angeles starting pitcher Carlos Frias.
After two foul balls evened the count, Upton stepped out of the box, looked at his bat, tapped it against his foot and determined it just didn't sound right. So Upton retreated to the visiting dugout to fetch another bat.
"I think the other one had a little crack in there," he said. "… I wasn't taking that one back up there."
One pitch later, Upton jumped all over a Frias fastball and drove it over the wall in center field for a grand slam and a 4-0 lead. Using the same bat, Upton added a two-run double in the second inning, giving the Padres a early cushion as they rolled to a 11-3 victory at Dodger Stadium.
"He's swinging the bat great. Right from the get-go, he's had good at-bats. He's a threat. He's doing what he does," said Padres manager Bud Black of Upton.
With the offense scuffling -- the Padres entered Sunday's game with just three runs in their previous four games, all losses -- Upton's grand slam was embraced by all.
"He's been taking the boys and putting us on his back, for sure," said Padres starting pitcher James Shields, who improved to 6-0 with the victory. "We're going to need some of the other guys to step up as well, to win more ball games, but he's been phenomenal. He's an All-Star-caliber player and doing his job."
Performances like the one Upton had on Sunday were the reason why the Padres traded for him in December, even if they knew there was a good chance the would-be free agent could play just one season for the Padres.
Upton leads the team with 12 home runs and 35 RBIs. He's had success at pitcher-friendly Petco Park. He's hitting .284, has a .343 on-base percentage and a .562 slugging percentage.
The grand slam was the second of the season for the Padres, who got one from Wil Nieves on April 12 against the Giants.
This was also the fourth career grand slam for Upton and his third against the Dodgers, the others coming in 2009 and 2011 while a member of the D-backs.
"I think we're a very good offense. It's just sometimes a little lack of focus and not getting good pitches. You see what happens when we get good pitches to hit," Upton said.
They got plenty of them Sunday.
The Padres finished with 20 hits, tying their season-high. They had a combined 18 hits in their previous four games -- all losses -- entering Sunday.
"Early in the year, we showed signs of some offensive production. Since then it's been variable. But we have that [Sunday's production] in us. You have seen that before, the big number. We have to do it again and again," Black said.
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.