Who's the next big thing in tennis?

Last season was a tough one for ATP prospects. The big prizes went to mid-career veterans, while up-and-comers like Juan Martin del Potro, Marin Cilic, Ernests Gulbis and Thiemo de Bakker fizzled for one reason or another. Now another group of fresh faces is making some noise, signaling this could be a better year for young contenders. We take a look at the five hottest 21-and-unders, led by last week's San Jose titlist, Milos Raonic.

1. Milos Raonic

He sent out a sign of things to come in Montreal two years ago, getting into the main draw as a qualifying wild card and holding a match point against Fernando Gonzalez before nerves took hold. Things went largely quiet until he burst on to the scene in Australia last month, reaching the second week as a qualifier and leading the tournament in aces. Last week, the 6-foot-5 20-year-old from Toronto blasted his way to his first ATP title in San Jose, vaulting himself up to the head of the current list of prospects on the tour.

A turning point came after Raonic qualified for the U.S. Open last summer and, tired from his efforts, fell in the first round. He vowed never to lose a match because of fitness again and began working with Spanish coach Galo Blanco in October to toughen him up physically. Raonic also has improved mentally, even though he confesses he's still prone to fiery losses once in a while. With a 15-3 record this year, it looks like he's started to pull everything together and is up to No. 59 in the rankings. Now comes yet another test -- adjusting his mindset and his schedule to match his new status.

2. Richard Berankis

This 20-year-old from Lithuania was the highest-ranked player under 21 until Raonic defeated him in the quarterfinals of San Jose and leapfrogged him this week. The two couldn't be more of a contrast, with the 5-foot-9 Berankis whizzing around the court with explosive strokes that belie his small frame. He's qualified for the previous three Grand Slams and won rounds each time. Now he'll start to get directly into big tournaments and should become a bigger name -- even though he's recently shortened it from the original "Ricardas." It only remains to be seen whether he can bring down the very biggest guns.

3. Kei Nishikori

It feels like this 21-year-old has been around for a while, having won his first tour title in Delray Beach all the way back in 2008. He became an overnight sensation in Japan after the win, but his progress in the past couple of seasons has been slowed by injuries, and he's currently ranked No. 68.

Another small player at 5-foot-10, his speed and rock-solid groundstrokes nevertheless make him very tough when he's playing well. He also faces the question of whether he has enough firepower to defeat the very best, but marathon performances against del Potro and Cilic at recent U.S. Opens have showed his competitiveness. Now working part-time with Brad Gilbert, he needs another big result to put him back on the map.

4. Grigor Dimitrov

The most-hyped of this bunch and arguably the most talented, "Baby Federer" has been putting in the hard yards at challenger events and has clawed his way into the top 100. He won three straight challengers after the U.S. Open, no easy feat, and reached another final and semifinal, losing to Nicolas Mahut and Berankis, respectively.

The 19-year-old from Bulgaria seems to have received a boost after starting to work with coach Peter McNamara last summer. But like many talented players, he can be mentally inconsistent and could take a little longer to make a big push.

5. Ryan Harrison

The 18-year-old Harrison is the baby of this group, and still well outside the top 100 at No. 142. But he recently won a strong challenger event in Hawaii and has been consistently winning matches at that level. After his breakout mini-run at the U.S. Open, Harrison cemented his position as the next big thing in American tennis by winning the USTA wild-card playoffs for the Australian Open, but then lost tamely in the first round.

Though mature beyond his years, his all-around game isn't quite ready for the big time just yet, and he's searching for a new coach after parting ways with Martin Damm. Getting into the top 100 this year would be a good target.

Honorable mention

Bernard Tomic. The 18-year-old Australian reached the third round of the Australian Open last month and always seems to deliver at home, but has had trouble producing the best of his skillful stroke play during the day-to-day rigors of the circuit.

Kamakshi Tandon is a freelance tennis writer for ESPN.com.