After improving one round on each of his previous four trips to Melbourne Park, odds are it will be top-ranked Nadal, who finished off a 6-2, 7-5, 7-5 win over No. 6 Gilles Simon on Wednesday as the temperature dipped to 93 from a daytime high of 109 degrees.
Nadal will meet another Spanish left-hander for a spot in the final after Fernando Verdasco ousted 2008 runner-up Jo-Wilfried Tsonga 7-6 (2), 3-6, 6-3, 6-2.
"I think it's incredible for us," Nadal said. "One will be in the finals, so we have to be happy with that."
No Spanish man has won the Australian title. Nadal reached the semifinals last year without dropping a set but was upset by Tsonga. He rebounded from that to win the French Open for the fourth time in a row, then ended Roger Federer's five-year reign at Wimbledon and his 237-week stretch at No. 1.
Verdasco had his time in the sun in November, when he guided Spain to victory in the Davis Cup final at Argentina while Nadal was recovering from knee tendinitis. Verdasco has now topped his best previous run of a fourth-round exit at a major.
Wednesday marked the start of what forecasters were predicting would be a once-in-a-century heat wave for the city. Hardy trees accustomed to a decade of drought were wilting. Dead or dying moths flopped onto the courts.
Nadal, a Majorca native, was relieved he'd been given a night match, and he joked about burning his feet when he went outside to practice.
"Believe me, I never feel the same like today when I was warming up outside," he said.
Federer, chasing a record-equaling 14th Grand Slam singles title to match Pete Sampras' career record, was playing American Andy Roddick in the night semifinal.
No. 2 Federer, who is 15-2 against Roddick and 6-0 in Grand Slams, lost in the semifinals to Novak Djokovic last year. No. 7 Roddick beat Djokovic in the quarterfinals Tuesday.
No. 14 Verdasco became the lowest ranked of the semifinalists when he beat Simon and is hoping to replicate Tsonga's '08 run.
Nadal owns a 6-0 record against him, including a French Open quarterfinal last year when Verdasco won only three games. But Nadal, 22, has seen vast improvement in Verdasco, who reached the final in Brisbane in a tune-up event and has now won five consecutive matches on hard courts for the first time.
His five-set win over Andy Murray, considered by many as a tournament favorite after recent wins over Nadal and Federer, grabbed attention.
"Fernando is playing at his best level," Nadal said. "I never played against him when he's playing at the level like right now, because I think he never played at this level before."
Verdasco said the Davis Cup victory, when he had to rally from 2-1 down to win the deciding singles match, was a turning point.
"That Davis Cup, it change my life so much and gave me a lot of confidence," the 25-year-old said. "Mentally made me much stronger for these matches here, five-set matches, and also with a lot of pressure, because it's a Grand Slam.
"That (win) in Argentina makes me grow up a lot."
Until then, he was more famous for posing nude for a magazine to promote men's health.
Verdasco did some off-season work with Gil Reyes in Las Vegas and had a two-hour discussion with Andre Agassi, a four-time Australian Open champion.
"For me, Rafa is the toughest player in five-set matches ... (and) is going to be the toughest match possible," Verdasco said.