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7/23/2014 1:15 A.M. ET

Barrett continuing to gain confidence in bullpen

DENVER -- The deeper the run in a pennant race, the more pressure situations arise.

Nationals manager Matt Williams knows this well from a playing career that included five postseason trips and a World Series victory with the Diamondbacks in 2001.

It's why he was so encouraged by the performance of reliever Aaron Barrett on Monday. With the bases loaded with two outs in the sixth and the Nationals clinging to a 4-2 lead, Barrett struck out pinch-hitter Brandon Barnes with a nasty slider in the dirt.

The Nationals added three insurance runs from there to take over sole possession of first place in the National League East.

Williams said Barrett has continued to gain confidence in similar tense situations, a development that can only serve Washington well as playoff races intensify.

"It's important for everyone involved," Williams said, "for his confidence and certainly for our team. I think one of the first ones he had was against [Miami Marlins slugger Giancarlo] Stanton early in the season. He's faced some middle-of-the order guys in some high-leverage situations and has done a good job. He's accustomed to it now. He's not afraid to take chances."

Williams said it helps that Barrett has enhanced trust in his out pitch, the hard-biting slider.

Barrett's successful appearance made a winner out of starter Doug Fister, who was impressed with the rookie reliever's approach in his first outing at Coors Field.

"That was huge," Fister said. "He came in and got the out. He made three big pitches that counted and that's the kind of trust you have in your bullpen, and that's what they do. They come in tough situations and get it done."

Zimmerman strains hamstring, will undergo MRI

DENVER -- An RBI groundout from Ryan Zimmerman came at a price Tuesday.

The Nationals third baseman beat out a double-play throw to first on an infield grounder in the sixth inning, but as he made his final stride, he suffered a right hamstring strain.

Zimmerman will undergo an MRI on Wednesday morning, Nationals manager Matt Williams said after the team's 7-4 win over the Rockies.

"It's concerning," Williams said. "Any time you have to leave the game with a hamstring injury, its concerning."

Zimmerman appeared to grab at the hamstring just before he reached first base, awkwardly tumbling over the bag just ahead of the throw.

Williams and Nationals head athletic trainer Lee Kuntz hurried out to the fallen third baseman and walked with him as Zimmerman slowly limped off the field, careful not to apply too much weight to the injured leg.

Zimmerman's hustle to the bag resulted in a run, with Anthony Rendon crossing the plate to cut the Colorado lead to 4-3.

The Nationals tied the score later in the inning on a single by Wilson Ramos, but the immediate concern was with Zimmerman, who entered Tuesday's game hitting .286 with five home runs and 35 RBIs in 52 games.

Zimmerman, through a team official, politely declined to discuss the injury until Wednesday, when he figured to have more information on his prognosis. Williams said the third baseman was "pretty sore" after the game.

Zimmerman missed 44 games earlier this season with a fractured right thumb, and he has occasionally dealt with a troublesome right shoulder, which was operated on in 2012.

His teammates were left hoping for the best after Zimmerman's most recent injury.

"He's a guy just coming back from the [disabled list] and was getting his swing back, so you hate to see that," said Adam LaRoche, who hit a three-run home run in the seventh. "We'll cross our fingers here and hope it's nothing major."

Desmond named Nats' Heart and Hustle Award winner

DENVER -- Nationals fans who watch Ian Desmond dirty his uniform with hard slides on the basepaths and dives in the infield won't be surprised by the honor bestowed upon the shortstop Tuesday.

Desmond was that Nationals' recipient of the 2014 Heart and Hustle Award, presented by the Major League Baseball Players Alumni Association for players who "demonstrate a passion for baseball and best embody the values, spirit and tradition of the game."

Desmond, who tied a career high with five hits Monday in the Nationals' 7-2 win over the Rockies, also was Washington's winner of the award in 2010, 2011 and 2013.

As the season draws to a close, a winner from the pool of 30 players -- one from each Major League team -- will be selected. Red Sox second baseman Dustin Pedroia won the award last season.

The final winner will be announced at the 15th Annual Legends for Youth Dinner in New York City on November 18.

To learn more about the MLBPAA, visit www.baseballalumni.com.

Fister, Frandsen both OK after twisting ankles

DENVER -- The Nationals may be starting to catch breaks on the injury front that weren't falling their way earlier this season.

Right-hander Doug Fister and utility player Kevin Frandsen suffered twisted ankles during Monday's 7-2 win over the Rockies, but outside of some general soreness, both players were fine Tuesday.

Fister twisted his ankle in the sixth inning as he maneuvered to avoid a hit up the middle by Carlos Gonzalez. He exited the game later in the inning with two outs and the bases loaded.

"I tweaked it a little bit, but I'll be fine," Fister said. "It will probably be sore for a day or two."

Frandsen twisted his ankle during a pinch-hitting appearance in the ninth inning. He caught a spike in the dirt, then rolled his left ankle while making contact for a groundout.

Nationals manager Matt Williams said there was some initial concern about Frandsen injuring his left ankle because he suffered a ruptured Achilles tendon in the same region earlier in his career.

"That's the same ankle," Williams said. "So any time there is an issue there, it scares him, scares us. If you have another issue with that Achilles, it's not good. But he showed up today feeling good. It's sore, but he's OK."

Williams said Frandsen was available off the bench Tuesday.

The Nationals may have avoided their biggest injury scare in the seventh inning Monday. Ryan Zimmerman, headed home from second after a Wilson Ramos single, tumbled safely across the plate as Rockies catcher Wilin Rosario failed to corral the throw from Gonzalez.

As Zimmerman rolled behind the plate, he knocked over Ian Desmond, who was waiting on deck. After several moments kneeling in the dirt behind the plate, Zimmerman walked unharmed into the dugout.

Nick Kosmider is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.