The Galicians can count themselves lucky to emerge with a point but they managed to cling on to a draw thanks to a first half penalty save from keeper Jose Molina and the home side's failure to put the finishing touches to some promising attacking moves.
The result leaves Depor three points clear at the top, but chasing duo Valencia and Real Madrid will have a chance to draw level if they win their matches on Sunday.
Fourth-placed Barcelona were unable to prove they could manage without the injured Ronaldinho when they slipped to a 2-1 defeat at Villarreal, whose victory was engineered by former Barca players Juan Roman Riquelme and Sonny Anderson.
Riquelme, who was dispatched to Villarreal on a loan deal in August after an unhappy first season at the Nou Camp, tormented the Catalan defence throughout the first half and supplied the corner that set up Jose Mari's headed goal five minutes before the break.
Although Patrick Kluivert equalised 18 minutes from time with a simple close-range finish, Anderson, who left Barca in 1999, grabbed an injury-time winner with a spectacular diving header after a great cross from fellow Brazilian Juliano Belletti.
The result means that Barca are now seven points behind leaders Depor and could lose fourth spot if Atletico Madrid or Osasuna win their matches on Sunday.
Last season's runners-up Real Sociedad slumped to their first home defeat in over 20 months when Cameroon striker Samuel Eto'o scored a sharply taken first half goal to give Real Mallorca a 1-0 win at Anoeta.
TORRENTIAL DOWNPOUR
Large swathes of the Betis pitch were swimming in water following a torrential downpour in Seville shortly after kickoff and neither side was able to settle into a rhythm in a disjointed opening quarter.
But the home team, who had gone into the game on a three-match losing streak, were given a golden opportunity to snatch the lead when defender Juanito won a penalty after he tangled with Molina in the area just before the half-hour mark.
The Depor keeper made amends, though, when he managed to beat away Fernando's poorly struck spot kick and keep his side on level terms.
Betis kept up the pressure but, with the conditions making their usual passing game almost impossible, they created danger only when free kick expert Marcos Assuncao sent a dipping shot over the bar on 33 minutes.
They continued to dominate after the break and had the ball in the net on the hour mark but pocket-sized forward Dani was denied a goal when team mate Martin Palermo was penalised for dangerous play for his acrobatic overhead pass.
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