Ivo Karlovic and Marin Cilic won five-set marathons at home to put Croatia 2-0 ahead of 32-time champions United States, while Israel stunned favoured Russia to also lead 2-0 in the Davis Cup quarter-finals on Friday.
Champions Spain were held to 1-1 by Germany in Marbella, and the Czech Republic and Argentina were tied after the opening two singles in their best of five tie.
Karlovic put hosts Croatia 1-0 up after coming back from two sets down to sink James Blake after three hours and 38 minutes in front of a partisan crowd in Porec.
"I was a bit nervous at the start, playing before the home crowd, but I became more relaxed as the match went on, and I managed to turn it around. This is surely one of the biggest matches of my career," Karlovic said.
Blake was unhappy with the home crowd's reception, saying some went too far even for a Davis Cup match.
"To boo our fans is something that I find unacceptable, to yell out while I'm serving, to clap when I miss a first serve and things like that on a pretty consistent basis seemed pretty hostile to me," Blake said.
Marin Cilic then capped off a perfect day for the hosts, beating a gallant Mardy Fish, who replaced injured Wimbledon finalist Andy Roddick.
Israel are within sight of a first semi-final at a major sporting event, with Dudi Sela and Harel Levy emerging heroes in front of more than 10,000 fans in Tel Aviv, the biggest crowd to watch a tennis match in the country.
Sela came from a set down to beat Mikhail Youzhny after Levy accounted for Igor Andreev, ranked almost 200 places above him.
AMAZING RESULT
"It's an amazing result. I didn't believe that after the first day we would have the two points," Sela said. "Hopefully, tomorrow is going to be the third one."
Last year's runners-up Argentina fell behind to Czech Republic in Ostrava when Tomas Berdych recovered to beat Juan Monaco in five sets but Juan Martin Del Potro levelled the tie at 1-1 with a comfortable straight sets win over Ivo Minar.
The Spanish, missing the injured Rafael Nadal and David Ferrer, had Fernando Verdasco to thank for putting them in front against Germany, although the world number nine had to come back from 2-1 down against Davis Cup newcomer Andreas Beck.
In sweltering conditions on the clay at the bull ring in Marbella, an efficient Philipp Kohlschreiber then brushed aside Tommy Robredo in straight sets to make it 1-1.
"I really played a very good game today with perfect tactics," said Kohlschreiber, who had never beaten Robredo in four previous meetings.
"I played maybe a little bit more aggressive and I moved him very well and waited for the short ball," he added. "I never missed a forehand in the whole match and that was really the key today."
The winner of the Spain-Germany tie will play Israel or Russia for a place in the final with the US or Croatia taking on Argentina or the Czech Republic.