W2W4 this fall: Halep? Kerber? Osaka? Anyone's guess who will end season ranked No. 1

The remarkable parity in the upper reaches of women's tennis these days has many upsides, one of them being the fall season has become that much more important. For the second consecutive year, the No. 1 ranking could be decided at the WTA Championships in Singapore.

The chance of that happening received a tremendous -- if unwanted -- boost Tuesday, when No. 1 Simona Halep announced she has a herniated disc in her back and is out indefinitely. Halep has a relatively slim lead of just more than 1,600 points on Angelique Kerber in the race.

Here are some of the main storylines in this last month of the WTA year to watch (men's can be found here), with ESPN tennis analyst Pam Shriver's take on each:

The favorite to snag the No. 1 ranking is ... TBD

If you're wondering why there's so little daylight between most of the elite players in the rankings, bear in mind a different woman has won each of the past eight Grand Slam tournaments.

Most of them, including Caroline Wozniacki, Garbine Muguruza, Petra Kvitova and Sloane Stephens, have struggled with consistency this year, which makes a triumphant tear through the final weeks of the season by any of them anyone's guess. It does open the door for Naomi Osaka, who won the US Open and has risen to No. 3 in the all-important race standings. Osaka just reached the final in Tokyo.

Shriver's take: "It's great that the race for No. 1 will go down to the wire, especially because a strong move by Osaka as the year ends could take her to the top of the game almost overnight. But take a good look at Kerber. She learned a lot in the year she struggled after becoming No. 1. Now she's back in the mix. She has the experience, and she's now teamed up with one of the best coaches [Wim Fissette] in the game."

Serena Williams' absence won't affect hierarchy down the stretch

There is a good chance we won't see Williams until 2019; she has been a chronic no-show in the fall. She has played the WTA Finals just three times in the past decade.

Williams, 37, still has an aura of greatness, but now that she's ranked No. 17, that glow of invincibility has faded. Familiar rivals and newcomers have picked up the slack, helping make this one of the most compelling years on the WTA Tour.

Shriver's take: "A casual viewer might be disappointed if Serena isn't in one of these tournaments, but to dedicated tennis fans, this abundance of contenders vying for the top ranking right up to the end of the year is really exciting. It is a shame that Serena hasn't been playing in the tour championships. I understand why, but it still doesn't look that good for the tour."

No post-US Open blues for Naomi Osaka

The most crucial question that has popped up on the heels of first-time Grand Slam tournament winners is, "Can they back it up?" The general pattern has seen great achievements followed by great letdowns. Look no further than Stephens' slump one year ago at this time, when she dropped six straight matches to end 2017.

For that reason, Osaka's run to the final in Tokyo was encouraging. But as the statisticians say, that's insufficient data. The coming weeks will tell us more.

Shriver's take: "Naomi has been a streaky player, as her results after the first big win of her career [at Indian Wells] demonstrated. Now she seems to have set a higher bar for herself, and so far she's clearing it. Naomi lost just three games to Dominika Cibulkova in her first match in Tokyo. That told me a lot."

It's go-time for some veteran champions

Do veteran Grand Slam champions Halep (if she plays), Wozniacki, Kerber and Kvitova have sufficient motivation and enough energy to keep the other Grand Slam champs of past two years -- all over whom are 25 and under -- at bay?

Kvitova had a 13-match winning streak (with a title at Madrid) on clay only to lose in the first round on her preferred grass at Wimbledon. She never fully recovered. Wozniacki has been very up and down after earning her elusive first major, in Australia. Kerber has been consistent but hardly great.

Shriver's take: "The veterans are like cars: Some are good for 200,000 miles; others break down at 100,000 or less. It gets your attention that Halep has a herniated disc at 27. Both tours need to look at the injury issue. It's hard to get going late in the year when the wear and tear is so severe. It makes it that much harder for veterans to step up at the end of the year."

The one understudy to watch

With so many new Grand Slam champions and so much parity on the tour, today's understudy may be tomorrow's star. The line separating the two has never been thinner. But doing great things in October and qualifying for the WTA Finals are two very different things. These championships have produced just one surprise champion in recent memory: Cibulkova, in 2016.

But pay attention to Aryna Sabalenka, whose stock has zoomed in the eyes of many experts since the US Open, where her fourth-round match with Osaka was a highlight of the tournament. Sabalenka, a winner at Wuhan last week, is up to No. 16 in the rankings, but the players from No. 8 to 16 are bunched closer than three tennis balls in their plastic sleeve. She could qualify for Singapore.

Shriver's take: "Sabalenka has the talent, the hunger and the strength to be a top player. But she's playing a crazy busy schedule [26 events and counting] and needs to assess the wisdom of that."