The old adage goes something like, "Baseball is a marathon, not a sprint." 

Indeed, a 162-game season ensures players will go through peaks and valleys, slumps and hitting streaks. Finishing strong is more important for both players and teams; just ask anyone on the 2019 Washington Nationals. 

That said, fast starts can precede stardom. Conversely, some players never quite recover from sluggish beginnings. How should these performances be rated, and how might early success—or lack thereof—project down the line?

Let's take a look at some of the most stunning starts to the 2021 season. The list will focus explicitly on players and will include those who have produced well beyond general expectation. It will also include a few whose campaigns have been more woeful.

Yermin Mercedes, Chicago White Sox

Yermin Mercedes has been in professional baseball since 2011. He did not get his first MLB at-bat until 2020, which was his lone at-bat of the season. But Mercedes has filled a massive void in the South Siders' lineup after Eloy Jimenez was lost to a torn pectoral. 

The 28-year-old made an enormous impact in his first two games, hitting safely in each of his first eight at-bats and racking up six RBI and three extra-base hits. He would run his hitting streak to six games and provide more fireworks with a 485-foot homer of Kansas City Royals starter Brad Keller on Thursday.

Mercedes' success (.556/.571/.889) is hard to pin down. On the one hand, he ranks in just the 22nd percentile in average exit velocity and 27th percentile in hard-hit rate. Mercedes also ranks in just the 22d percentile in chase rate. 

Judging off batted-ball numbers alone, it would seem regression is likely. But it isn't that simple. Mercedes ranks in the 94th percentile in max exit velocity and 84th percentile in barrel rate. He can give it a ride and consistently gets the barrel through.

The other important consideration is Mercedes was a .302/.366/.491 career hitter in the minors. He also ranks in the 91st percentile in whiff rate, which is suggestive of his hit tool. 

The numbers are unsustainable, but he has hit all along the way and only seems to be getting more comfortable at the dish. Mercedes could be an impact bat for the White Sox.

Tyler Naquin, Cincinnati Reds

Tyler Naquin never quite lived up to the promise he showed in his rookie season in Cleveland. But he sure is off to a fantastic start in Cincinnati.

Naquin has already surpassed his 2020 total with five homers through his first seven games. He has nearly as many walks (4) as he did in 40 games last year (5). The resulting slash line of .318/.444/1.000 has helped power a Reds offense that has also seen prolific performances from Nick Castellanos and Jonathan India, among others. 

Nothing about Naquin's start has been fluky. He ranks atop MLB in both average exit velocity and hard-hit rate. He also ranks in the 99th percentile in barrel rate and 95th percentile in max exit velocity.

Is this the year the 29-year-old really breaks out? Naquin's .632 OPS in 2020 might be slightly deceptive given he ranked in the 87th percentile in average exit velocity and 93rd percentile in hard-hit rate. The fact that he has maintained strong batted-ball numbers early is quite encouraging, especially now that he is playing in a more hitter-friendly environment at Great American Ball Park. 

The concern with Naquin is the swing-and-miss tendencies. He ranks in the 14th percentile in whiff rate, which doesn't bode all that well for how he'll fare once pitchers start making adjustments. It will also be interesting to see whether Jesse Winker digs into his playing time.

Still, Naquin could be a necessary run-producer with even greater upside if he can continue to draw walks and gets enough at-bats.

Cedric Mullins, Baltimore Orioles

The Baltimore Orioles had a surprise in store when they swept the Boston Red Sox in their opening series. Outfielder Cedric Mullins was red-hot against Boston. 

Mullins racked up nine hits in three games, including a five-hit, three-double performance in the series finale. He epitomized the notion of creating steady traffic on the bases, consistently giving teammates the opportunity to drive in runs.

The 2015 13th-round pick has since cooled just a touch. But Mullins hit safely in each of his next four games, also clubbing a home run in an extra-innings win over the New York Yankees on April 7. He then had two more hits against the Boston Red Sox on Saturday.

Mullins will not be a big power guy. But he projects quite well as a line-drive hitter who can put the ball through the middle of the field. The 26-year-old has a 40.9 percent line-drive rate and ranks in the 70th percentile in average exit velocity.

It should be noted Mullins ranked below the 50th percentile in average exit velocity and was in just the 18th percentile in hard-hit rate in 2020. He also had a ground-ball rate over 50 percent in each of his first three seasons. 

Regression seems likely. However, it is an inherently good sign Mullins' line-drive percentage is so high considering he makes a good deal of contact.