This is, as far as I know, nearly always the case in France, which uses a two-round system for almost all elections (including the regional elections, where it does not make much sense, but not the elections to the European parliament, where it's not possible).
The second round is widely seen as the one that matters. The results of the first round do change perceptions, influence negotiations between political parties and also determine how much public funds a party gets in the following years but who will make the threshold to reach the second round is often not in doubt so that the only thing deciding who gets elected is the result of the second round of voting (certainly for the presidential election - with one major exception, obviously - other elections are a bit more complex).
I could not find a convenient table tallying up voter turnout over a long period of time but I have checked the 2007 and 2012 presidential elections, 2004 and 2010 regional elections, and 2004 local elections on the official election website of the Interior ministry and the stats confirm my recollection.
For presidential elections, the difference isn't huge because turnout is already high and possibly also because many people still vividly recall what happened in 2002 (in France, you just need to say "April 21" without any other qualification and everybody who is old enough and cares at least a bit about politics should know you are speaking of the results of the 2002 presidential election). For regional elections, the difference can be up to 5 percentage points, which means over 2M people (out of population of 66M people, with slightly more than 46M registered voters).
EDIT: I found a Wikipedia article confirming this for the presidential and legislative elections. Note, however, that the table for the legislative, local (cantonales), and municipal elections are not directly comparable as a candidate or a list who gets more than 50% of the vote in the first round of voting is elected directly, which automatically depresses turnout in the second round (and, in municipal elections, many lists run unopposed because France has literally tens of thousands or really tiny municipalities).