Simona Halep's No.1 defense kicks-off immediately in 2018, as Garbiñe Muguruza and Caroline Wozniacki aim to chase her down during the opening week of the 2018 season. WTA Insider breaks down the No.1 scenarios.

The WTA No.1 ranking will be in play during the opening week of the 2018 season, which begins on December 31st. No.1 Simona Halep, No.2 Garbiñe Muguruza, and No.3 Caroline Wozniacki all have a chance to head into the Australian Open as the top seed and World No.1 pending the results at their respective opening tournaments next week. Seeding at the Australian Open will be based on the WTA Rankings on Monday, January 8th.

The 2017 season saw an historic battle for No.1 unfold, with five different women holding the top spot over the the season. The Battle for No.1 came down to the wire last season, with Halep enjoying a late surge to win the Porsche Race to Singapore and edge out Muguruza by just 40 points. Wozniacki capped off one of the best seasons of her career by winning her biggest title at the BNP Paribas WTA Finals Singapore presented by SC Global, to finish inside the Top 5 for the first time since 2011.

Current No.1 Halep kicks off her season at the Shenzhen Open (International level), where she was a champion in 2015 but lost in the Round of 16 last season. Muguruza is the top seed at the Brisbane International (Premier level), where she was a semifinalist last year, and Wozniacki returns to the ASB Classic in Auckland (International level), where she was a quarterfinalist last year.

Related: Where the Top 30 will begin their 2018 seasons

Here's how the scenarios shake out:

- Halep will remain at No.1 if she wins the title in Shenzhen.

- Muguruza will need to reach the final in Brisbane in order to have a chance at returning to No.1.

- If Muguruza wins the title in Brisbane, then Halep would need to win the title in Shenzhen in order to remain at No.1.

- If Muguruza finishes as runner-up in Brisbane, Halep would need to reach the semifinals in Shenzhen in order to stay at No.1.

- If Muguruza finishes as runner-up in Brisbane and Halep is a semifinalist in Shenzhen, they would both have the same ranking points. However, Halep holds the tie-breaker and would therefore remain at No.1, as she has more points among WTA Finals, Grand Slams, Premier Mandatory and Premier 5 events over the last 12 months (Halep: 5,636 vs. Muguruza: 5,396).

- Wozniacki is looking to return to No.1 for the first time since January 2012. She needs to win the Auckland title, with Muguruza losing before the final in Brisbane and Halep losing before the quarterfinals in Shenzhen.