CHICAGO -- The first hit was a check-swing RBI single in the first inning. The rest were more vintage Mike Piazza.
"When you get a broken-bat hit or half swing, it gives you momentum. Your confidence is picked up a little bit," Piazza said.
Piazza doubled in the go-ahead run in the fourth. And the Padres broke the game open with five runs in the fifth, thanks to three errors by the Cubs, who have dropped 12 of 14. Friday's loss was their fifth to the Padres in a week.
San Diego's Dave Roberts extended his hitting streak to 10 games, going 3-for-4 and scoring three runs. Brian Sweeney (2-0) allowed one run in three innings of relief, after starter Woody Williams strained his left calf running the bases in the second.
Williams had an MRI after the game and was headed to the disabled list.
"He's going to be out a while," manager Bruce Bochy said. "We just don't know for how long. We're going to talk about what we're going to do from here."
Williams walked with two outs in the second inning and came up limping as he approached third base on a ground-rule double by Roberts. Clay Hensley pinch-ran for Williams, and Mike Cameron struck out to end the inning.
Sweeney relieved Williams, who allowed one run and two hits.
Chicago's Glendon Rusch (1-5) gave up six runs -- five earned -- in 1 2/3 innings of relief.
"It was the result you expect from a day when you don't make quality pitches," Rusch said.
Piazza, batting .220 entering the game, drove in Roberts with a double to left with two outs in the fourth to give the Padres a 3-2 lead. Piazza's last four-hit game was last season with the Mets on May 18 against Cincinnati.
The game turned ugly for the Cubs in the fifth.
Vinny Castilla led off with a single, and things unraveled from there. Adrian Gonzalez followed with a single, and shortstop Ronny Cedeno -- thinking he had called timeout -- threw the ball out of play toward the third base dugout, and the runners advanced to second and third.
With the bases loaded and one out, two runs scored when second baseman Neifi Perez threw wildly to first trying to complete a double play on Roberts.
Roberts wound up at second and stole third. He scored on Cameron's double and exchanged words with Cubs catcher Michael Barrett as he crossed the plate. Brian Giles then chased Rusch with a single that made it 7-2.
Scott Williamson relieved, and Piazza singled. Left fielder Matt Murton allowed Giles to score when he misplayed Khalil Greene's sinking line drive, before Castilla grounded out to end the inning.
"I can't really comment on the whole matter right now," Barrett said of his exchange with Roberts. "It was a really frustrating loss."
Roberts, who thought Barrett was angry at him for stealing third, insisted he was not trying to show up the Cubs. He also said he has "nothing but respect" for Barrett.
"I respect those guys over there," Roberts said. "I've known Michael for a long time and he's a heck of a player. I just think they're frustrated over there. They're not playing the baseball they want to play."
Gonzalez hit a two-run homer in the eighth for the Padres to make it 10-4.
In their first game back at Wrigley Field after a 1-8 trip, the Cubs heard it from their fans. Rusch was serenaded by boos as he left the game. But there was plenty of blame to go around.
There was no shortage of frustration in the clubhouse afterward.
Cubs starter Angel Guzman walked five batters, hit one with a pitch, threw a wild pitch and allowed two runs in three innings. The rookie has walked 16 batters in 18 innings in four starts since being called up from Triple-A Iowa.
"It's pretty tough for me," he said. "I have to find that release point and be consistent with it."
The Padres placed catcher Rob Bowen on the 15-day disabled list with a sprained left thumb and recalled outfielder-first baseman Paul McAnulty from Triple-A Portland on Friday. The club also recalled right-handed pitcher Jon Adkins from Portland and sent RHP Dewon Brazelton outright to Triple-A. ... Bochy brushed aside Rick Sutcliffe's rambling and slurred interview during the local San Diego telecast of Wednesday's game against Milwaukee in which his microphone was apparently cut off. "It's not like he cussed," Bochy said. "Lighten up, everybody." ESPN issued an apology from Sutcliffe on. ... With a soggy field after a day of rain, the Cubs kept RF Jacque Jones (groin) out of the lineup. Jones is batting .340 with three homers and eight RBI in his last 14 games.