Q:

When was the last time a single country had three players in Top 10 of WTA rankings?

If Karolína Plíšková wins her today's match against Angelique Kerber, she will move into the 10th spot in the next edition of WTA singles rankings. (Sources: wtatennis, tennisworldusa, idnes, ...) If this happens, there will be three Czech players in Top Ten of the rankings. (The other two being Kvitová and Šafářová.)
To me it seemed a rare occurrence. (I do not remember something like that in the recent years.) Which provoked me to ask the question:

When was the last time a single country had three players in Top 10 of WTA singles rankings?

I have tried to check at least year-end rankings. It is probably not an unexpected that I found several years where some big nations (Russia, USA) had three or more players in top 10. However, my question is not only about year end rankings.

Russia had 3 or more players in year-end rankings in 2009 (Safina, Kuznetsova, Dementieva, Zvonareva), 2008 (Safina, Dmentieva, Zvonareva, Kuznetsova, Sharapova), 2007 (Kuznetsova, Sharapova, Chakvetadze), 2006 (Sharapova, Kuznetsova, Petrova, Dementieva), 2005 (Sharapova, Dmentieva, Petrova)
In 2004 there were four players from USA (Davenport, Serena and Venus Williams, Capriati) and four players from Russia (Myskina, Sharapova, Kuznetsova, Dementieva) in Top Ten.
USA had three or more players in Top 10 in 2003 (Serena Williams, Davenport, Capriati, Rubin), 2002 (Serena and Venus Williams, Capriati, Seles), 2001 (Davenport, Capriati, Venus and Serena Williams, Seles), 2000 (Davenport, Venus Williams, Seles, Serena Williams)
In 1999 there were four players from USA (Davenport, Venus and Serena Wiliams, Seles) and four players from France (Pierce, Tauziat, Halard-Decugis, Mauresmo) in Top 10.

A:

I don't know whether this was the last time, but it was the last time I was able to find.
On 24 May 2010 there were three Russian players in Top 10 (Elena Dementieva, Svetlana Kuznetsova and Dinara Safina.)
This can be checked on WTA website. (But I do not know how to link to rankings from some given date.) The Wikipedia article on 2010 French Open shows ranking from 17 May 2010. (These rankings were used to determine seeding at this tournament.) You can see there these three players in Top 10.
However, it is hard to say whether this was the last time when this happened. If somebody knows a better method how to look for statistics like this, I would be glad to know about it.

