Nick Kyrgios, king of 'tweeners, comes up big at the Miami Open

Nick Kyrgios has made myriad headlines for his boorish behavior, but he is a sublime shot-maker. At 6-foot-4, he is athletic and unafraid. In the fifth game of the opening set Thursday at the Miami Open, he hit not one, but two casual between-the-legs shots in a single point -- and won it when his rattled opponent, Alexander Zverev, blew an easy forehand volley into the net.

Oh, and then in the first game of the second set, there was an unbelievable running 'tweener crosscourt forehand winner. But on Zverev's third set point in the second, Kyrgios tried another ill-advised 'tweener -- and failed.

Kyrgios is a 21-year-old Australian who is unable to resist the seemingly pathological urge to hit flashy shots. As for the buttoned-down 19-year-old German, Zverev? He had a momentary lapse of composure when he stepped on his racket and broke it in half.

Ah, youth.

The two first played at this elite level all of 16 days ago in the third round at Indian Wells. On Thursday, they reprised the sparkling yin-yang matchup that could captivate us for the next decade.

Kyrgios, who is one week from being a full two years older than his opponent, is ahead in their development so far, which was evident down the stretch in their quarterfinal clash on Key Biscayne.

The No. 12-seeded Kyrgios handled No. 16 Zverev 6-4, 6-7 (9), 6-3. The Aussie clobbered 16 aces and was never broken. Oh, and the final (unofficial) 'tweener count: five all told, three successful. One, incredibly poorly timed.

"To be honest, it felt good," Kyrgios told the media afterward. "It just felt like we were both competing as hard as we could. I thought we both served like really well today. ... Honestly, in the match I wasn't thinking about anything but just competing and trying to work him out tactically."

Kyrgios reached his second straight semifinal in Miami, and Zverev, making his Masters 1000 quarterfinal debut, has now lost to him twice this month.

Waiting in Friday night's semifinal? Who else but 35-year-old Roger Federer, who earlier needing three sets to send home Tomas Berdych.

This one may not be the layup you think, Fed Fans.

Kyrgios has a 1-0 head-to-head advantage, but it's marginal. Their only match was a three-tiebreaker affair two years ago in Madrid.

"Big players -- they like the big stage," Federer said of Kyrgios and Zverev, before he knew which one he'd face. "Both have a big serve. Both real nice backhand, and the belief, so that makes them just very tough to play, you know in any given match.

"I both like their game, and I think they're going to be very, very good players moving forward. They already are, but I'm talking big picture."

It's a riveting meeting that offers a glimpse of the past and future. Earlier this month, Kyrgios had advanced to the quarterfinals at Indian Wells and was scheduled to face the resurgent Federer when food poisoning intervened, and the Aussie had no choice but to grant the 18-time Grand Slam champion a walkover.

"He's the greatest of all time," Kyrgios said afterward. "He's my favorite tennis player, so I'm going to enjoy that moment. We don't know how long he's going to hang around."

Some context from this zany #NextGen match: This was the youngest quarterfinal, semifinal or final at an ATP Masters 1000 event since 2008, when Novak Djokovic (21) beat Ernests Gulbis (19) in the Cincinnati quarterfinals. A year earlier, Djokovic and Andy Murray, both teenagers, made the quarters in Miami. Zverev, who turns 20 next month, is the youngest player to reach the quarterfinals in Miami since.

They are undeniably dashing and madly skilled. And in some cases, particularly Kyrgios, unafraid to show swagger.

"I like to entertain the crowd a lot," Kyrgios said in his on-court interview. "I don't really know what I was thinking."

Zverev, at 6-6, is more buttoned-down, but he has huge weapons and a backhand that goes beyond his years. Still, it was Zverev who drew a code violation by methodically breaking his racket after an early error in the second set.

"I'm a competitor," Zverev said. "That's my job. I mean, I will always leave everything out there, like I did today. Sometimes you come up short."

Nonetheless, the ATP World Tour is teeming with young talent.

Teenagers Frances Tiafoe and Taylor Fritz, as well as 20-year-olds Karen Khachanov, Borna Coric and Hyeon Chung already have outsized games. But the two emerging players that excite tennis aficionados the most are Zverev and Kyrgios They are the youngest players in the top 20.

Kyrgios beat Rafael Nadal at Wimbledon as a teenager, and this year he has taken down Djokovic in back-to-back tournaments. Last year, Zverev upset Federer on the grass and has now beaten three-time major champion Stan Wawrinka twice.

"It's pretty surreal for me still," Kyrgios said. "I was watching Gael Monfils on TV. I went to Jo-Wilfried Tsonga's practices when I was 12 years old. So it is still pretty surreal to see those guys in the locker room and me being next to them in the locker room, playing them, beating them.

"Yeah, I mean, Roger. I mean, it was only four years ago where I was watching him on TV. I mean, yeah, it's a weird feeling obviously being out there and competing against him."

Everyone has to start somewhere.

"I think it will be exciting regardless of what happened in Indian Wells," Federer said. "I think it's nice to play him again. I predict that [Zverev] is going to be a great player. Nick has already proven his point now."

In 2007, Murray and Djokovic first arrived as teenagers in the Miami quarterfinals. A decade later, they are the Nos. 1 and 2 players in the world.

Get used to seeing Kyrgios and Zverev -- they are the future of Grand Slam finals. Zverev just needs a little more time to mature, mentally and physically.

If Kyrgios can learn to produce a consistent effort, we'll finally be able to speak of him in the context of major titles. But for now, 'tweener talk will have to do.