ISTANBUL, May 26 (Reuters) - The Turkish lira weakened to a record low just off 20 against the dollar on Friday ahead of this weekend's presidential election runoff which will decide whether President Tayyip Erdogan extends his rule into a third decade.

At 0413 GMT, the lira stood at 19.9845 against the dollar, off a record low of 19.9950 in early trade.

Turkey's sovereign dollar bonds and equities have plunged, while the cost of insuring exposure to Turkish debt has spiked since the first round of the presidential election on May 14.

Erdogan was well ahead of his main rival Kemal Kilicdaroglu in that vote and fell just short of the more than 50% support he needed to avoid going to Sunday's runoff. (Reporting by Daren Butler; Editing by Muralikumar Anantharaman)