Barca’s star signing of the close season Lewandowski now has six goals in five Liga matches.
The match was halted in the 82nd minute because of a presumed heart attack in the stands. Cadiz ‘keeper Jeremias Ledesma grabbed a defibrillator from the team bench and sprinted across the pitch to throw it to emergency staff in the stands.
The fan was taken away by ambulance.
“This sort of thing goes beyond football, we’re talking about a human life here, so we all agreed to stop,” coach Xavi said.
“Fortunately they managed to save him.”
Cadiz later said the fan had been treated by on-site medics who had their own defibrillator.
Barcelona scored twice more late on, through Ansu Fati on 86 minutes and Frenchman Ousmane Dembele, who made it 4-0 in stoppage time.
“It wasn’t a great game, but it was a good one,” said Xavi, who takes his team to Bayern Munich for a Champions League match on Tuesday.
Atletico Madrid beat Celta Vigo 4-1 in an open match to climb into the top four on 10 points.
Argentine striker Angel Correa put them ahead after nine minutes.
He was set up by compatriot Rodrigo De Paul, who scored the second on 50 minutes.
Antoine Griezmann came on shortly after the hour and was soon celebrating with fellow substitute Yannick Carrasco who made it 3-0.
Celta pulled one back through Gabriel Viega Novas before an own-goal from Unai Nunez on 82 restored the three-goal gap.
“We took risks and it worked,” said Atletico coach Diego Simeone. “If they (Celta) had many chances it’s because we played it that way, you have to do that sometimes.”
Sevilla won 3-2 away at Espanyol for their first victory of the season thanks to a double from Jose Angel Carmona and one goal from Erik Lamela, who also collected a late red card as the visitors hung on for three points.
Sevilla now go to Copenhagen for the Champions League in mixed spirits.
“At 3-0 we sat back after a good first half, but their goals were a reminder of recent failures,” said coach Julen Lopetegui.
Rayo Vallecano beat Valencia 2-1 helped by an own goal from Nicolas Gonzalez. Mouctar Diakhaby scored a last gasp consolation for the visitors.