Lithuania's presidential election rules require a candidate to win a majority of votes to claim the post in the first round. However, if turnout falls below 50%, a second provision requires a winning candidate to claim at least 33% of all voters. Article 74 of the Law on Presidential Elections notes:
6. A candidate to the office of President of the Republic shall be considered elected if during voting for the first time in which at least half of all voters participate, he receives more than half of the votes of all voters participating in the elections. If less than half of all voters participated in the elections, a candidate to the office of President of the Republic shall be considered elected when he receives the most, but no less than one-third of votes of all voters.
As of 2pm, turnout was 26.57% nationally. In a polling station I visited today in Vilnius, turnout was around one-third at 3pm. The prospect of exceeding 50%, with polls closing at 7pm, seems grim. If turnout is low, the likelihood of a second round increases substantially.
UPDATE (11:oo pm Vilnius Time): Apparently, Lithuanians prefer to vote in the afternoon. The final preliminary turnout is just a hair under 50% - 49.69%. If these results are certified, a second round is almost a certainty. If the final turnout creeps above 50%, Grybauskaite will win in round 1. ANOTHER UPDATE: My interpretation of the turnout data was slightly errant. The precincts reported one hour before closing.