Serena Williams and Angelique Kerber are the first two women to qualify for the BNP Paribas WTA Finals Singapore presented by SC Global. The World No.1 and No.2 respectively, this marks the fourth consecutive year Serena has qualified for the Finals and the second straight year for Kerber. While their paths have crossed twice in their quest for Singapore, their roads have looked markedly different.

Though they’ve only faced off twice this season, the Williams-Kerber rivalry has defined the 2016 season. Their two meetings came on tennis’ biggest stages, in the final of the Australian Open and Wimbledon, with Kerber winning in Melbourne and Serena in London. Kerber’s consistent success at tour events has also allowed her to close the gap on Serena’s lead in the rankings, which seemed all but insurmountable a year ago. Serena currently has seven zero-pointers on her ranking, having missed the Asian Swing and WTA Finals last fall.

Serena’s season has been the more surgical of the two. She is 34-5 on the year, having made the final or better at five of the six tournaments she has played (not including the Olympics). She won two of them, Rome and Wimbledon. By winning at the All England Club in July, Serena matched Stefanie Graf’s Open Era record of 22 major singles title. At the upcoming US Open, where she will attempt to break the record, Serena will also match Graf for the most consecutive weeks at No.1, having spent 186 straight weeks in the top spot. After a near-historic season in 2015, the records just keep on coming for one of the game’s greatest of all time.

Kerber has been the workhorse this year, which comes to no one’s surprise given her reputation as one of the hardest workers in the game. After making the final of the Western & Southern Open this weekend, she now leads the tour with 47 wins in 2016. She has played 16 tournaments this season, making the final of five them, and won two titles at the Australian Open and Stuttgart. But it hasn’t just been about finals for Kerber. She’s consistently putting herself into the final four of the tour’s biggest events, all while also playing two rounds of Fed Cup for Germany:

Brisbane – Finalist
Australian Open – Champion
Miami – Semifinalist
Charleston – Semifinalist
Stuttgart – Champion
Wimbledon – Finalist
Montréal – Semifinalist
Olympic – Finalist (does not count towards qualification)
Cincinnati – Finalist

In all, Kerber is 7-3 against Top 10 opposition this season. No other Top 10 player has even had that many matches, let along equaled Kerber’s wins.

Current Top 5 vs. Top 10 opposition.

Serena: 5-2
Kerber: 7-3
Muguruza: 3-2
Radwanska: 2-3
Halep: 2-2

As the tour heads to the final Slam of the season at the US Open, Serena and Kerber are separated by less than 500 points in the Road to Singapore. This past weekend, Kerber came within one win of becoming the second German ever to become World No.1. Kerber will challenge for the No.1 ranking in New York as well.

Given how close the rankings are with just a few months left in the season, there’s a strong possibility that the coveted year-end No.1 ranking could come down to the wire at the WTA Finals in Singapore.