The Finn recorded four stage wins to open up a 39.7s lead before Tanak responded by winning the final test of the day to reduce the margin to 30.8s heading into Sunday’s final three stages.
Championship leader Sebastien Ogier ended the day third, 40.2s adrift of Rovanpera, but only 9.4s behind Tanak, while Hyundai’s Dani Sordo is in no man’s land in fourth position.
The M-Sport Ford pair of Adrien Fourmaux and Gus Greensmith were locked in battle for fifth and will head into the final day split by 15.8s.
Elfyn Evans recovered from from Friday’s gearbox problem to climb from 16th to seventh (5m51.9s), while Thierry Neuville surged from 18th to 10th after his mechanical dramas yesterday.
Rovanpera recorded his fourth consecutive stage win to underline his stunning pace with victory on Stage 10.
The Toyota driver was fastest by 6.5s from teammate Ogier but took an eye-opening 18.3s out of Tanak, who reported no unusual issues on the stage to explain the time drop.
Despite being caught out be a tight left-hand hairpin towards the end of the test, Neuville and Evans finished with third and fourth-fastest times to continue their rise up the leaderboard.
Shortly after the final WRC runner had completed the test, organisers red flagged and then cancelled the stage due to spectators being in dangerous locations.
Greensmith was lucky to avoid a sizeable crash after a wild moment on the test as he continued to hunt down his teammate Fourmaux.
Stage 11 was a repeat of the day’s first test where Rovanpera shocked the field with his blistering pace.
However, on the second pass he produced a much more reserved run as the streak of stage wins was halted by Ogier. The Frenchman who had earlier claimed “it would not be wise for us to take a lot of risks” set the benchmark time which was 1.1s faster than Tanak, who recovered some time lost on the previous stage.
The result saw the championship leader close to within 2.5s of Tanak for second overall.
Rovanpera dropped 6.2s to Ogier before revealing at stage end that he’d struggled with the extra weight from carrying two spares wheels.
“Not happy with this stage. It was difficult with two spare wheels – a bit more difficult than I was expecting to be honest,” said Rovanpera. “In the slow and narrow parts it felt a bit lazy.”
Sordo fell further towards the battling M-Sport Fords of Fourmaux and Greensmith in the minor placings after dropping 21.7s to Ogier on the stage.
The final stage of the day belonged to Tanak, who looked a different driver compared to his first pass through the roads that hosted Stage 10.
The Estonian, aided by soft rubber and a set-up tweak, was 3.8s faster than Rovanpera, while Ogier clocked a time 3.1s slower.
“I managed to do some changes which finally helped something,” said Tanak. “I felt the car a bit more playful now and the tyre choice was good.”
In WRC2, Marco Bulacia reclaimed the lead in the class from Andreas Mikkelsen on the final stage of the day, only 0.8s separates the pair.
Yohan Rossel leads WRC3 by 19.1s from Kajetan Kajetanowicz with Britain’s Chris Ingram third.
The Acropolis Rally concludes with three stages on Sunday, with the first beginning at 0508 GMT.