The new year is just a week old, but some of the main storylines for 2019 are already fraying, making the question of what 2019 really holds in store that much juicer. Here are some of the trends detected after the first week of play:

The Big Four reunion might be a bust

In December, fans salivated over the return of the Big Four. Just a week into this season, we're wondering if Andy Murray and Rafael Nadal will even be able to contend at the Australian Open (never mind beyond).

The entire quartet was scheduled to play during the first week of January, but the only one to come away with a win was Roger Federer. His appearance at the Hopman Cup, where he faced off with Serena Williams in a mixed-doubles match made in heaven, was a grand spectacle. The Swiss team (Federer and Belinda Bencic) won it all, but at the end of the day it was just an exhibition.

Once again, Rafael Nadal heads into a major event with health concerns. Saeed Khan / AFP)SAEED KHAN/AFP/Getty Images

Top-seeded Novak Djokovic was upset by Roberto Bautista Agut in a high-quality semifinal at Doha, Qatar. In an echo of the bad old days of 2017, at one point Djokovic smashed a racket in frustration, and he turned frosty with the media afterward: "What happened?" he repeated. "I lost the match. That's it."

Nadal, ranked No. 2, and Murray, down to No. 240 because of time missed with his persistently bad hip, were both entered in Brisbane, Australia. Murray, whose movement is still compromised, was blasted off the court in the second round by late-blooming Next Gen bombardier Daniil Medvedev. Nadal, hoping to be fully fit for the Australian Open, pulled out to nurse what he described as "a small strain in my left thigh." Want a sobering stat? Nadal has completed just one of the past 19 hard-court events he's entered.

And the latest WTA contender is ...

Young stars Naomi Osaka, Elina Svitolina, Jelena Ostapenko and other familiar champions (Sloane Stephens, Petra Kvitova and Caroline Wozniacki among them) failed to hit the jackpot in Week 1 -- some were bounced from tournaments shockingly early. Veteran Karolina Pliskova, 26, came through with a title at Brisbane while 30-year-old Julia Goerges triumphed in Auckland.

But the hottest player in the WTA since last fall has been Aryna Sabalenka, and the way she mowed down the field at Shenzhen last week suggests that the 20-year-old from Belarus has the makings of the dominant force that the WTA has been lacking since Serena Williams left for maternity leave.

Sure, we've been down this road before, with other strong young women who had bold, powerful games and early success (Garbine Muguruza, Ostapenko and Kvitova). None of them has had staying power at the top. But eventually, someone will emerge to rule the roost. Sabalenka seems ready to make her bid.

The Next Gen shine is fading

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Nobody likes a buzzkill, but it's time for a reality check. Are the young pros who comprised the original Next Gen group destined to be Grand Slam champions? Top 10 contenders? Top 30? Journeymen?

Alexander Zverev, 21 and ranked No. 4, stands head and shoulders above his Next Gen peers (only Karen Khachanov has joined him as the winner of a Masters 1000 or better). Three others in the group are ranked in the next 10 (No. 11 Khachanov, No. 12 Borna Coric, and No. 15 Kyle Edmund). The other original members are Hyeon Chung (No. 25), Nick Kyrgios (No. 35), Yoshihito Nishioka (No. 75), Elias Ymer (No. 115),Thanasi Kokkinakis (No. 146), Quentin Halys (No. 128), Taylor Fritz (No. 49) and Frances Tiafoe (No. 39).

These are mostly good, solid players. Some of them have stalled because of injury. Others are being eclipsed by younger players (including No. 15 Stefanos Tsitsipas and No. 27 Denis Shapovalov) or late bloomers (No. 16 Medvedev).

If the original Next Gen group is going to distinguish itself as a generation, it has to do it soon. Zverev looks like the favorite, but a hamstring injury he suffered in practice forced him to pull out of the World Tennis Challenge in Adelaide on Monday.

Injuries once again will play an outsized role

Zverev, Nadal and Murray aren't the only ailing stars. The sight of Williams grabbing her right shoulder on several occasions during the US mixed-doubles team's duel with the Swiss squad sent alarm bells ringing, even if she played down the significance: "It was such a quick turnaround [from her previous match]," Williams told reporters. "I didn't have enough time to reload the cannon. It's totally normal."

More disconcerting, however, is the injury suffered by Maria Sharapova, who abandoned her Shenzhen quarterfinal this past week while trailing Aryna Sabalenka because of a thigh injury. It was Sharapova's first tournament since the US Open.

No. 1 Simona Halep will play this week for the first time since September, when she was sidelined with a herniated disk, while former US Open finalist Madison Keys has struggled with wrist and knee injuries.

Juan Martin del Potro will miss the Australian Open while continuing to recover from a fractured patella, and former Wimbledon finalist Milos Raonic can't seem to get healthy. The good news: Oft-injured No. 9 Kei Nishikori was healthy enough to win Brisbane on Sunday.

Bigger might finally be better

The year began with a final featuring the greatest combined height of finalists ever in an ATP event as 6-foot-8 No. 6 Kevin Anderson outlasted 6-foot-11 ace machine Ivo Karlovic in the Pune, India, final. Was it a harbinger?

Historically, the Big Four (tallest member: 6-foot-3 Murray) has held off the crush of big men who have filtered into the game. But if they falter, all bets are off. Djokovic, Nadal and Federer are ranked Nos. 1, 2, 3, respectively, but then it's (in order) Zverev, del Potro, Anderson and Cilic. Every one of them is 6-foot-6 or taller. Incidentally, so is Khachanov, and then there's No. 10, 6-foot-10 John Isner.

That should leave 5-foot-7 Diego Schwartzman and all the smaller players with plenty to worry about.