The 23-year-old Swede defeated Finn Emil Ruusuvuori 6–3, 6–7, 6–4 in the third round of the Washington ATP tournament.
It was 36 degrees and scorching sun over the outdoor track in the American capital.
Ymer comfortably won the first set, in the second the players followed each other and Ruusuvuori won the tiebreak with 7–3.
In the third, when the players were clearly affected by the heat, they broke each other's serve game several times.
Ymer was at a disadvantage for a long time, but then went from 3–3 to 5–3 and finally won 6–4 after almost three hours of play.
In the quarter-finals he will face Grigor Dimitrov, Bulgaria, or Sebastian Korda, USA.
On his way to his third quarter-final this year, his first since January, the 115-ranked Swede has knocked out former world number one Andy Murray, now ranked 50, Russia's 36-ranked Aslan Karatsev and now the 46-ranked Finn Ruusuvuori.
TENNIS: Mikael Ymer's heavy scalp - knocked out the former world number one (August 2)
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TENNIS: Mikael Ymer's heavy scalp - knocked out the former world number one