Suninen set the pace from the get-go and chalked up five consecutive stage wins to gain an early advantage in his Hyundai i20 N Rally2.
But, as the rough and rocky stages chewed through the field, the Finn suffered a rear right puncture on SS7 and dropped almost a minute.
Mikkelsen was left almost 40sec clear following Suninen’s demise, although he also had problems as his Škoda Fabia Rally2 developed a mystery misfire.
The engine was losing power and sounded more sick by the kilometre, and while Mikkelsen was able to coax the car back to service in Matosinhos 37.0sec ahead of Suninen, he later retired the car.
Suninen’s lead was restored once more and he went to bed with a healthy 17.1sec advantage over Yohan Rossel’s Citroën C3 Rally2.
"Our day was actually pretty good,” said Suninen. “Our pace has been on the top, it’s just unfortunate that we got the puncture on a hairpin. I don't know what I could have done differently, but that's rallying," he added.
Mikkelsen was devastated to lose out on what could have been his third win in as many rallies, but told WRC.com that he plans to restart on Saturday.
“We had engine issues since the afternoon,” he explained. “We were struggling with a lack of power and running on three cylinders.
“We tried what we could, but the team said it’s better to retire the car here because there’s no way they would have time to fix it in a normal service.
“It’s a shame because we were driving sensibly and we had built up a big gap,” he added.
Suninen’s Hyundai colleague Oliver Solberg was 23.0sec back from Rossel after struggling with dust in the cockpit as well as two punctures. He had 46.1sec in hand over Fabia driver Kajetan Kajetanowicz, who dropped time stuck in the dust when Marco Bulacia stopped to change a wheel on SS6.
Polish star Miko Marczyk was fifth and led the WRC2 Junior contingent in his Škoda. He trailed the leaders by 1min 55sec at close of play, with Chris Ingram’s similar car 21.4sec behind after puncturing on SS7. Frenchman Jean-Michel Raoux led the WRC2 Masters category in his Volkswagen Polo GTI R5.
Jan Solans and Georg Linnamäe both finished more than six minutes off the pace. Solans was nursing a faulty differential on his C3 and Linnamäe also reported a fraught day in his Polo.
Of the 40 crews that started, only 25 made it back to service. Eric Camilli dropped out on Thursday with an electrical fault on his C3 before SS1, while Eerik Pietarinen’s Polo went out with a burst brake pipe on SS7.
Other retirements included i20 N hotshot Josh McErlean with broken steering and Erik Cais, whose Ford Fiesta lost a wheel.
- Full coverage from Vodafone Rally de Portugal is available on WRC+ All Live here, including every stage broadcast as it happens as well as key interviews, features and expert analysis from the service park.
Finland
Italy
Hungary