SAN DIEGO -- The Cubs were piecing together a complete game on Monday night. The offense chased Padres starter Yu Darvish from the Petco Park mound after three innings and poured out eight early runs. Righty Javier Assad was cruising for Chicago, putting on a show in front of a host of family and friends.

It was a formula for an easy win, but baseball is famous for flipping the script. Things spiraled at a rapid rate for the North Siders, leading to a 9-8 loss that was a category of defeat that Chicago had not experienced in more than two decades. It had been 22 years since the Cubs had an eight-run lead slip away in a loss.

“Momentum is a real thing,” Cubs shortstop Dansby Swanson said. “They were obviously able to capture it and we weren't able to kind of push back.”

The final blow arrived in the eighth inning, when Fernando Tatis Jr. -- one pitch after being brushed back by Cubs closer Adbert Alzolay -- drilled a slider to left field and admired his blast before flipping his bat. The two-run shot gave San Diego its ninth run, overcoming the 8-0 advantage the Cubs had built through the first four frames.

Cubs lefty Luke Little, who yielded a two-run homer to Xander Bogaerts in the sixth inning, was 2 years old the last time his ballclub saw that large of a lead disappear in a loss. That was on June 28, 2002, when the Cubs were up, 8-0, on the White Sox, only to lose, 13-9.

“We just couldn't stop it anywhere, really,” Cubs manager Craig Counsell said. “It was just one of those nights where they had some good at-bats and we couldn't make the next pitch and get the next out. And they made us pay for it.”

The Cubs flew to their eight-run lead by scoring four runs in each of the second and third innings. In the second, Chicago forced Darvish to toil through 42 pitches in a long, draining inning that featured two-run singles from Ian Happ and Cody Bellinger. In the fourth, Swanson capped off the next four-run burst with a two-run triple off the wall in right.

Assad used the cushion to his benefit, compiling five scoreless innings that included a few howls and fist pumps as he notched three inning-ending strikeouts. With a wide lead, Counsell felt it was appropriate to give the right-hander a chance to take the ball to start the sixth.

“Javy did a really nice job,” Counsell said. “We sent him out for the sixth, hopefully, just to get a couple outs. And we didn't get that.”

Tatis drew a walk to open the inning and then Jake Cronenworth ended a nine-pitch battle by launching Assad’s 104th pitch of the evening out to right field for a two-run blast. At that juncture, Counsell emerged from the dugout and summoned sidearmer Jose Cuas from the bullpen.

“He pitched an amazing game,” Cuas said of Assad. “He deserved to win that game. Nothing else I can say about that.”

In a span of three pitches, the Padres had seized their momentum.

First, Manny Machado pulled a sinker from Cuas with a 96.8 mph exit velocity to Swanson’s right at shortstop. Swanson has made a living and earned two Gold Gloves with his skills on defense, especially with snaring grounders with backhand snags. This time, the baseball eluded his grasp and skipped into the outfield.

“You can kind of think back through like, ‘Should I have done this? Should I have done that?’” Swanson said. “At the end of the day, I just didn't make it, you know? One that you wish you had back. You wish you could obviously take away that opportunity.”

Jurickson Profar sent Cuas’ second pitch into right field for a single. The Cubs reliever then fired a sinker to Ha-Seong Kim, who drove that third pitch of the outing into right field, where it got by outfielder Mike Tauchman and resulted in a two-run triple.

“They jumped him really quick, essentially,” Counsell said. “Three of them were first pitches, so they were just super aggressive with him.”

Luis Campusano added a run-scoring groundout and Jackson Merrill singled, ending Caus’ night. That set the table for Bogaerts’ blast to extend San Diego’s sixth-inning rally to seven runs. Tatis took care of the rest.

“'It's a good learning experience for us, for this group and for everyone involved,” Swanson said. “At the end of the day, it counts as one, right? It stinks. No one wants to win more than us. But sometimes, things happen.

“And so much of this game is about, it's not necessarily what you do, it's how you respond.”