Roberto Martinez's side have taken 12 points from 12 on offer during an unblemished start to proceedings, while their hosts are only two points worse off and could usurp their more esteemed rivals with a maximum this week.
Match preview
On the hunt for a third successive appearance at the European Championship finals, Slovakia - who progressed to the last 16 in 2016 before a group-stage elimination at the delayed 2020 edition - have cemented their position in the top two of Group J as widely expected.
A goalless draw at home to Luxembourg represented a solid if not sensational start to their latest qualifying venture, but Francesco Calzona's side have since come up trumps against Bosnia-Herzegovina, Iceland and Liechtenstein, conceding just one goal along the way.
The manner of their victories against Iceland and Liechtenstein was hardly emphatic, though, as they edged past the former 2-1 before relying on a header from defender Denis Vavro to get over the line against Liechtenstein - a nation ranked 153 places lower than them.
Results will certainly take precedence over performances for head coach Calzona, though, as his side sit three points clear of Luxembourg in the second qualifying spot, and there will be no Nations League reprieve for the world's 47th-ranked nation should they drop out of the top two.
A seven-game unbeaten run will stand Slovakia in good stead, and the Falcons have managed to keep clean sheets in each of their last four European qualifying contests, but not since 2015 have they strung together a four-game winning streak across all tournaments.
Intent on spoiling the Slovakian party, Portugal boss Roberto Martinez had big shoes to fill when Euro 2016-winning boss Fernando Santos was lured to Poland, but the former Everton and Belgium head coach could hardly have wished for a better start to life in the dugout.
From their four opening fixtures with Bosnia-Herzegovina, Luxembourg, Liechtenstein and Iceland, Portugal have accumulated 12 points and 14 goals while keeping the back door bolted shut at the other end, amassing four clean sheets to boot.
It was hardly a vintage display from the Selecao when they made a stopover in Iceland three months ago, but on his 200th appearance for the national team, Cristiano Ronaldo made the net ripple for the 123rd time in international football to propel his side to a 1-0 triumph.
Only France and Hungary can also boast perfect defensive records in Euro 2024 qualifying, while their total of 14 goals scored makes them the second-most prolific nation in the groups behind England (15), and their overall winning streak in Euro qualification matches stands at a respectable six.
From their last 19 matches at this level, Portugal have suffered just the one defeat - away to Ukraine in 2019 - and they have already come through four meetings with Slovakia without suffering defeat, most recently prevailing 2-0 in a 2006 World Cup qualifier during which Ronaldo scored the Selecao's second of the afternoon.
Slovakia Euro Champ Qualifying form:
D
W
W
W
Slovakia form (all competitions):
D
D
D
W
W
W
Portugal Euro Champ Qualifying form:
W
W
W
W
Portugal form (all competitions):
W
L
W
W
W
W
Advert - content below:
Team News
Few surprises have been sprung in the Slovakian selection for September's fixtures, where Paris Saint-Germain new boy Milan Skriniar and Newcastle United goalkeeper Martin Dubravka should marshal the backline as ever.
Still going strong at 36 with Slovakian giants Slovan Bratislava - whose home ground will host Friday's fixture - Juraj Kucka should win his 100th cap for the Falcons here, likely alongside Napoli lynchpin Stanislav Lobotka, while Marek Hamsik has now called time on his glittering career.
With 15 strikes to his name, Sydney FC winger Robert Mak represents the top goalscorer in the Slovakia ranks and should be given the nod in the left-wing position, but striker David Strelec is only among the reserves this time around.
While Slovakia have waved goodbye to a national team hero in Hamsik, 38-year-old Ronaldo continues to don the skipper's armband for Portugal and enters the international break having scored six times in his opening four league games with Al-Nassr this term, in addition to providing four assists.
Fellow Saudi Pro League representatives Otavio and Ruben Neves have also been selected, as have a pair of uncapped players in Wolverhampton Wanderers' Jose Sa and Toti Gomes, but their former teammate - Manchester City's Matheus Nunes - is missing.
Notable defensive absentees also include Raphael Guerreiro, Nuno Mendes and 40-year-old centre-back Pepe, while Benfica duo Rafa Silva and Joao Mario have both called time on their international careers.
Slovakia possible starting lineup:
Dubravka; Pekarik, Skriniar, Vavro, Hancko; Duda, Lobotka, Kucka; Suslov, Bozenik, Mak
Portugal possible starting lineup:
Costa; Pereira, Dias, A. Silva; Cancelo, Palhinha, Fernandes, Dalot; B. Silva, Ronaldo, Leao
We say: Slovakia 0-2 Portugal
Neither Slovakia nor Portugal have proven easy to break down in the embryonic stages of qualifying, and the Selecao will face their toughest test yet in Bratislava, despite making hard work of their recent success over Iceland.