Fourmaux reached Friday lunchtime service with a 4.6sec advantage over team-mate Thierry Neuville, but it was Solberg who produced the standout drive of the morning.
The Toyota Gazoo Racing driver recovered from tyre damage on the opening Tula – Erula test to win SS4 Monti di Alà – Sa Conchedda – Lerno by 0.8sec from Sébastien Ogier.
Solberg was a remarkable 16.5sec faster than championship leader Elfyn Evans on the 23.18km stage and climbed from fourth to third overall, 8.1sec behind Fourmaux.
More importantly for his title hopes, the Swede now sits 2.6sec ahead of second-placed championship challenger Sami Pajari and 16.8sec clear of Evans on the road.
“After the puncture this morning I just tried to drive normally again,” Solberg said. “It was a clean run, nothing special, but I just try to drive as good as I can really.”
Fourmaux had opened Friday with the fastest time on SS2 despite hanging dust affecting visibility, then inherited the rally lead one stage later when Ogier suffered a front-right puncture.
The Frenchman backed that up with third-fastest time on SS4 to retain control at the front.
“We really tried to be smooth with the driving in some places to avoid the risk of puncture,” Fourmaux explained after SS3. “There were so many bad rocks, loose rocks in some places, so it’s not easy. I’m happy with my pace and how I’ve managed the tyres for now.”
Neuville moved into second with a stage win on Su Filigosu – Lerno and remains firmly in contention, while Pajari sits fourth, 10.7sec from the lead.
The Finn was quicker than Evans on all three morning stages and admitted he was focused on maximising the opportunity presented by his later road position.
“At least I’m faster than Elfyn, but I’m a bit worried that it’s not enough,” Pajari said after SS4.
Evans endured the most difficult morning of the three championship contenders. Running first on the road, the Welshman faced the worst of the loose gravel and reached midday sixth overall, 24.9sec behind Fourmaux.
Ogier had briefly taken the rally lead after SS2 before his puncture on the following test cost more than 25sec. He recovered with second-fastest time on SS4 and sits fifth overall, 23.0sec from the front.
Punctures became a recurring theme as the morning progressed. Takamoto Katsuta, Jon Armstrong and Mārtiņš Sesks all stopped to change wheels on SS4, while Dani Sordo had already lost around four minutes changing a damaged tyre on SS3.
Josh McErlean emerged seventh overall after a consistent morning, while Teemu Suninen leads the WRC2 runners in eighth.