Looking to complete the rare four-game sweep, instead the Washington Nationals fell into a lot of the same mistakes that have plagued them all season. Missed opportunities. Sketchy defense. Questionable decisions. It all ended up in a 5-2 loss to the Cincinnati Reds on a dreary, rainy day before 36,457 at Nationals Park. The Nationals crossed the 500,000 mark in attendance in April with the big weekend series.

The Nats (13-12) biggest obstacle Sunday was Reds rookie pitcher Tony Cingrani. Making his third MLB start, the lefty was sublime, using a mid-90s fastball and sharp breaking ball to strike out 11 in six innings. The Nats managed just two hits off Cingrani and walked once.

Ross Detwiler, the Nats starter, was off for the first time this season and received “death by papercuts,” allowing four runs on 11 hits — 10 singles and four infield singles — in five innings to take his second loss of the season.

The Reds got into Detwiler right off the bat in the first inning. Zack Cozart singled to the opposite field on a 2-2 count with one out, followed by a rocket double to the left field corner by Joey Votto. Brandon Phillips then singled to center, scoring both runners.

The second inning saw more damage to Detwiler’s ERA. With one out, Cingrani reached when his ground ball hit the third base bag and eluded Anthony Rendon. Shin-Soo Choo grounded to Rendon to start what could have been a double play, but Danny Espinosa flat-out dropped the relay, so all hands were safe. Cozart drew a five-pitch walk, and Votto singled to plate Cingrani. It could have been worse, but Phillips lined into a 3-6 double play to end the inning.

Cincinnati picked up another run against Detwiler in the fourth. Catcher Corky Miller reached on an infield single, took second on the pitcher’s sacrifice, went to third on Choo’s ground out and scored on Cozart’s infield single. The Reds nickel-and-dimed Detwiler to death.

The Nats finally got on the board in the seventh. Ian Desmond singled on a bloop to right field. After Adam LaRoche struck out (0-for-25, 13 Ks in his last six games) Anthony Rendon drew a walk. Kurt Suzuki followed with a double into the left field corner that scored Desmond. But with one out and runners at second and third, the Nats wasted those other runners, as Steve Lombardozzi pinch-hit into a comebacker and Denard Span grounded to short to end the rally.

Manager Davey Johnson went to Henry Rodriguez for the eighth, and it cost his team a run. Hot Rod faced four batters, walking three of them. He threw 17 pitches, four of which were strikes. Ryan Mattheus entered with the bases loaded and Cozart greeted him with a sacrifice fly to score a run. Mattheus limited the damage by getting Votto to line out to end the inning.

The Nats got that run back in the bottom of the frame. With one out, Harper drew a walk on a 10-pitch at bat against Jonathan Broxton. One out later, Ian Desmond lofted a fly ball to right center that Shin-Soo Choo couldn’t handle. The ball kicked away and Harper scored from first, with Desmond getting credit for a double. Desmond was left stranded when LaRoche flied out for the final out.

Aroldis Chapman blew the Nats away in the bottom of the ninth for his sixth save of the season.

THE GOOD: Craig Stammen. Two perfect innings of relief with two more Ks.

THE BAD: Adam LaRoche: 0-for-5, 3 Ks. 5 LOB. Denard Span: 0-for-5, 4 LOB.

THE UGLY: Henry Rodriguez. 17 pitches, four strikes. Ino ne sequence, he almost hit Miller in the head, and bounced the next pitch in the left-handed batters box, which skipped to the backstop, ricocheted and almost hit Rodriguez at the pitcher’s mound on the rebound. (link courtesy @recordsandradio)

THE STATS: 5 hits, 4 BBs, 14 Ks. 1-for-7 with RISP, 8 LOB. E: Espinosa (2, catch), Rendon (3, throw). DP: 2.

NEXT GAME: Monday at 7:10 pm against the Atlanta Braves from Turner Field. Stephen Strasburg 91-4, 3.16) faces Julio Teheran (1-0, 5.48).