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Asked what about Bryce Harper impresses him, Mike Trout began with a stock-sounding answer.

"He plays the game hard. He's max effort every time," Trout said. Then a smile creased his face as he added, "besides the 'lack of hustle' the other day."

Ah, yes. That topic might not go away anytime soon.

Two of the most dynamic young players in baseball, Trout and Harper found themselves as opponents in the majors for the first time Monday night, when the Los Angeles Angels opened a three-game series at the Washington Nationals. The comparisons by fans started a while back — and might very well continue for years.

"If they like him, they like him. If they like me, they like me. If they like both of us, then they know the game," Harper said. "And if they don't, then they're crazy."

Washington's Harper is 21, Los Angeles' Trout is 22. Both are outfielders. Harper was the NL Rookie of the Year in 2012, when Trout won the AL honor. They were teammates in the Arizona Fall League in 2011 and have kept up a friendship, trading text messages occasionally and chatting before Monday's game.

"This series is not Mike Trout against Bryce Harper. Those guys are two guys that are in a different class of player than you're going to see going around the major leagues," Angels manager Mike Scioscia said. "And you get a chance to see them on the same field."

Harper is hitting .292 with one homer, five RBIs and 18 strikeouts this season. Trout, meanwhile, has a .307 average with five homers, 13 RBIs and 22 strikeouts.

Trout is the more all-around accomplished of the two so far, finishing second in AL MVP voting in each of his two full seasons to Detroit slugger Miguel Cabrera.

"He's one of the best players, if not the best player, in baseball," Harper said. "He's a lot of fun to watch."

As for whether it makes sense to compare Harper and Trout, Nationals manager Matt Williams said, "I don't think it's fair."

"Everybody brings a different set of tools to the table. Bryce is one of our main guys. We rely on him. And the Angels rely on Mike. The comparison's natural, I think, because of all that's been written and documented, their timelines, all that," Williams said. "It's probably a little unfair to compare them, because they're too very different players. They both have exceptional tools and exceptionally high ceilings. But to compare them, I don't think Bryce compares himself to Mike or vice versa. We certainly don't do that, either."

Trout doesn't mind being put in that sort of conversation, though.

"Around the same age, same hype — we were always going to be compared," he said.

Harper, for his part, said that while he doesn't try to see how he matches up with Trout, he understands why others will.

"I know I'm a ... good player, and he is too. We're going to roll through baseball over the next 20 years, hopefully, and make people turn their heads," Harper said. "He's going to do it, and hopefully I can do it."

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AP freelance writer Ian Quillen contributed to this report.

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Follow Howard Fendrich on Twitter at http://twitter.com/HowardFendrich