The loss broke the Nationals' modest three-game winning streak.
Washington was in a 10-run hole after one-half inning. Marquis faced seven hitters and didn't record an out, giving up seven runs on four hits. The right-hander became the first starter in Nationals history to not record an out in a game. Marquis threw 28 pitches, 13 strikes.
Marquis has now given up 19 earned runs in 8 1/3 innings for Washington this season.
During the game, manager Jim Riggleman wondered whether something was physically wrong with Marquis, but pitching coach Steve McCatty told the skipper that Marquis was OK.
After he left the game, Marquis went in the indoor batting cage and threw a lot of pitches to build up arm strength. According to McCatty, Marquis threw almost nothing but strikes and was able to get his sinker down.
"Jason is definitely healthy. That's the first thing I asked after he came out of the game," Riggleman said. "He is down in the dumps, of course, on how he performed out there today.
"He went down there and threw more and more down in the cage. That's not something you would do when you are physically not up to it."
Marquis even said he was healthy, but he doesn't have a feel for his pitches, which was a common theme for him during Spring Training.
"It's an embarrassment what I'm doing out there right now," Marquis said. "I have to find that answer quick. It's just not happening right now. I don't think there is anything behind the ball. I don't feel any power, any feel of a release point to get the ball where I want it right now. I have to find a way to get it done. I don't think my delivery is where I want it to be. I have no excuses. I just want to get it done. That's all it comes down to."
The Nats have not made a decision on Marquis' next start. He could pitch on short rest, skip a start in order to fix his mechanics or pitch on his regular turn, which would be Friday against the Dodgers. Meanwhile, Marquis and McCatty will look at video and see how they could fix Marquis' problems on the mound.
McCatty believes he is trying too hard and needs to have one good game to get him on a roll.
"Sometimes, it gets to be a mental thing where you are trying so hard," McCatty said. "I'm not saying that is necessarily the case. I just know he gives you a lot of innings, and he's a strike-thrower. He just needs to get on that roll and be nice and relaxed out there. That is the best way to put it."
Even Brewers shortstop Craig Counsell realized that something was not right with Marquis, noting that most of the sinkerballer's pitches were up in the strike zone.
"I don't think he had good command," said Counsell, who went 2-for-4 with a grand slam. "He was having trouble throwing strikes, more than anything. I don't think his velocity was any different. You could tell he really couldn't tell where the ball was going. He had trouble with that."
Reliever Miguel Batista struggled upon entering the game in the first inning, serving up the grand slam to Counsell -- the third of his career -- to give the Brewers a 10-run lead.
"It was a pitch where I missed my location," Batista said. "I did a good job, probably two hitters too late."
Batista settled down thereafter, turning in five innings of three-run ball, and the Nationals made it a game by the fifth inning against Brewers left-hander Doug Davis, who only lasted 4 2/3 innings.
Washington scored its first run in the second inning, when Adam Kennedy doubled down the right-field line to drive in Ivan Rodriguez.
Three innings later, the Nationals made it a five-run game by scoring four runs off Davis, with Ryan Zimmerman highlighting the scoring with a two-run single.
The Nationals made it a 10-7 game in the bottom of the seventh inning as Ian Desmond came in as a pinch-hitter and doubled down the left-field line to drive in two runs. But the Brewers increased their lead by four off left-hander Jesse English in the eighth inning, thanks to an RBI double by Jim Edmonds.
"We played well. It's a credit to our guys. The energy was good. The attitude was good in the dugout in terms of we are going to get back in this ballgame," Riggleman said.
But it wasn't so good on the mound for Marquis.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.