CINCINNATI -- Sitting in the visitors' dugout at Great American Ball Park earlier this week, Cubs president of baseball operations Theo Epstein was hesitant to talk at length about his team's torrid trip to the top of the National League Central. He knows how swiftly things can change along a baseball season's winding road.

"I don't want to take any victory laps or anything now," Epstein said. "We've played pretty darn good baseball, but we're entering a real tough part of the schedule now. This next month will represent a real test for us."

On Thursday night, Chicago's test was taking on emerging ace Luis Castillo, who set the tone for the Reds in what amounted to a 4-2 loss for the Cubs. A 1-hour, 51-minute rain delay in the sixth inning cost Castillo a chance at more innings, but Cincinnati's bullpen kept the North Siders' offense subdued and sent Chicago to its first series loss since April 5-7 in Milwaukee.

• Box score

Do not be fooled by the fifth-place Reds. They entered the evening with the Majors' second-best ERA as a team (3.35), ranking just ahead of the first-place Cubs (3.44). Next, Chicago will head to Washington, where Max Scherzer and Stephen Strasburg loom. Jake Arrieta and the Phillies are on deck, and the Astros, Cardinals and Dodgers are all coming in the near future.

“Everybody's doing a really good job so far,” Cubs lefty José Quintana said. “So, we'll try to stay on that page. I don't remember when it was, the last series we lost. We lost this one. Tough for us, but we'll keep going. We're in a really good position, so we'll try to keep winning games.”

Chicago's run of eight consecutive series victories (and 10 unbeaten series in a row) was bound to snap at some point. That stretch came after a 2-7 record to begin the season, helping the Cubs surge up the division standings at a more rapid rate than pundits thought possible in the wake of the tough start.

"The guys showed up really ready to play this year. They want it," Epstein said. "I think the team is gelling in a really nice way. They care about each other and are real together. There's real focus each day on doing what's necessary to put our best foot forward and try to win. That's something the players get credit for, and [manager Joe Maddon] and the coaching staff.

"It has to come together, and then you have to find a way to maintain it all year long through the ups and downs, especially this year given the small margin for error that every National League club's going to have. So, we didn't get out of the starting blocks the way we wanted to, but we found our stride. Now, it's a matter of really maintaining it."

That slim margin for error was on full display against Cincinnati, which took advantage of a pair of ill-timed wild pitches.

Quintana took the mound in the fifth armed with a 2-0 advantage. Two batters into the frame, Jose Peraza sent a pitch out to right field for a solo home run, cutting Cincinnati's deficit to one run. Castillo followed with a single to left and Joey Votto later added a base hit to center, creating a two-out situation with runners on first and second.

With Eugenio Suarez at the plate, Cubs catcher Willson Contreras positioned himself for what looked like a pitch up in the strike zone with a 1-2 count. With rain falling, Quintana slipped on the mound and misfired with a sinker that tailed low and outside, where it struck the dirt and eluded Contreras' mitt. Votto and Castillo sprinted to second and third base, respectively.

“He was sitting in a certain spot, and the ball was way in the other direction,” Maddon said. “We've got to be able to overcome those mistakes.”

Next, Quintana opted for a 2-2 curveball, which he sent tumbling to the dirt as well. Contreras was unable to get into a strong blocking position in time, allowing the baseball to skip through his legs and to the back wall for another wild pitch. Castillo hustled home and scored the tying run, as Quintana fell on top of him while trying to cover the plate.

“It's tough. It's frustrating that happened twice,” Quintana said. “One inning, and that changed my game.”

Suarez capitalized on the moment with a single to left that scored Votto, giving the Reds a 3-2 lead. After the rain delay knocked Quintana out of the contest, Cincinnati tacked on another run against Tyler Chatwood in the seventh.

That showing overcame the Cubs' lone breakthrough against Castillo (1.90 ERA on the year) in the first inning. Contreras chopped a pitch up the third-base line, where it struck the bag and popped over the head of Suarez. The ball dropped into left for a double, scoring a pair of runs for the Cubs.

“You have to beat good pitching. We have to pitch,” Maddon said. “We pitched pretty good ourselves tonight, but you've got to pitch better than good pitching. That's what we've got to do. We're not going to just bludgeon anybody that we're facing. We're facing too many good pitchers.”

And that plays into Epstein’s hesitance to reflect too much on the first quarter of the Cubs’ schedule.

“Those are good questions for the middle of November,” he said, “not in early May, to sort of look back on everything. It's a journey. The way we played the first road trip, I didn't think was indicative of our talent level or our preparation or our guys' focus.”