That the first two play before a final happens for various reasons.
The first time, in Palm Springs in 1977, it was a round-robin format.
The last four times, in the WTA Finals of 2010, 2012, 2019 and 2023, it was for the partial round-robin format, where the first seed from one round-robin group plays the second seed from the other round-robin group in the semi-finals and vice versa, meaning that No. 1 and No. 2 could easily clash in the semi-finals.
At Roland Garros in 1996, Steffi Graf and Monica Seles were ranked No. 1 at the time as Seles continued her return to the tour, and were the top two seeds there. Conchita Martínez, who was ranked No. 2 at the time, was seeded No. 3 and fell into Graf’s middle of the draw, and they met in a semi-final between Pussy. 1 and 2.
And then there have been several cases where the rankings were based on the previous qualifying period and No. 1 and No. 2 fell in the same half, like tonight.
