This time last year, the story unfolding in the 2018 WTA season revolved around the return of serve. Twelve months on and it's the serve that has set the current Top 10 apart from the field. 

Current WTA Top 10

1. Naomi Osaka (9-2)
2. Simona Halep (10-4)
3. Petra Kvitova (17-4)
4. Sloane Stephens (5-4)
5. Karolina Pliskova (13-3)
6. Elina Svitolina (9-4)
7. Kiki Bertens (10-5)
8. Angelique Kerber (7-4)
9. Aryna Sabalenka (12-4)
10. Serena Williams (4-1)

In 2018, the start of the season was dominated by counter-punchers and aggressive counter-punchers. Caroline Wozniacki, Angelique Kerber, and Elina Svitolina swept the biggest tournaments in Australia, before Petra Kvitova tore through February. This season has been about the power players, highlighted by Naomi Osaka, Kvitova, and Karolina Pliskova picking up the big titles Down Under. 

Champions at Premier or higher events: 2018 vs. 2019

Brisbane International
2018: Elina Svitolina (d. Sasnovich)
2019: Karolina Pliskova (d. Tsurenko)

Sydney International
2018: Angelique Kerber (d. Barty)
2019: Petra Kvitova (d. Barty)

Australian Open
2018: Caroline Wozniacki (d. Halep)
2019: Naomi Osaka (d. Kvitova)

St. Petersburg
2018: Petra Kvitova (d. Mladenovic)
2019: Kiki Bertens (d. Vekic)

Qatar Total Open
2018: Petra Kvitova (d. Muguruza)
2019: Elise Mertens (d. Halep)

Dubai Duty Free Tennis Championships
2018: Elina Svitolina (d. Kasatkina)
2019: Belinda Bencic (d. Kvitova)

So far, the Top 10 have outpaced the tour average in every key serving metric. 

The Top 10 is hitting an average of 2 aces more per match than the average rate. World No.1 Osaka leads the tour in aces per match, firing an average of 8, while the tour's outright ace leader, No.7 Kiki Bertens, is third on the list, averaging 7.47 per match.

Most notably, the Top 10 are holding serve nearly 10% more of the time than the tour average. Nine of the Top 10 have a higher percentage of Service Games Won than the tour average, with the lone exception being Sloane Stephens (64.6% vs. tour average of 66.3%). 

No.10 Serena Williams has posted the highest hold-rate of the Top 10 at 80.8% over 5 matches, with No.3 Kvitova right behind her at 80.6% over 21 matches. Along with Pliskova, Osaka, and Aryna Sabalenka, 5 of the current Top 10 feature in the tour-wide Top 10 leaderboard for service games won. No.12 Ashleigh Barty leads all players at 81.7%.

Current Top 10: Service Games Won - Tour Average: 66.3%

1. Serena Williams: 80.8%
2. Petra Kvitova: 80.6%
3. Karolina Pliskova: 79.8%
4. Naomi Osaka: 78.0%
5. Aryna Sabalenka: 77.9%
6. Kiki Bertens: 76.7%
7. Elina Svitolina: 74.6%
8. Simona Halep: 71.0%
9. Angelique Kerber: 69.2%
10. Sloane Stephens: 64.6%

While the tour's elite are winning nearly 10% more of their service games than the tour average, the difference on return is less significant. The current Top 10 are breaking serve at 36.1% of the time, just 2.4% more than the tour average of 33.7%.

Serena is the outright tour-leader in return games won, having broken 48.1% of the time in the five matches she played at the Australian Open, her only event of the season so far. No.6 Svitolina owns the second-best return games won percentage among the Top 10, breaking at 40.6% of the time. 

Current Top 10: Return Games Won - Tour Average: 33.7%

1. Serena Williams: 48.1%
2. Elina Svitolina: 40.6%
3. Aryna Sabalenka: 37.5%
4. Angelique Kerber: 36.4%
5. Petra Kvitova: 36.0%
6. Simona Halep: 35.8%
7. Karolina Pliskova: 35.7%
8. Naomi Osaka: 33.9%
9. Kiki Bertens: 32.8%
10. Sloane Stephens: 31.7%

Stats and information provided by SAP Tennis Analytics.