The Scot had earlier announced that he has made a decision to sit out next week's Rogers Cup in Toronto and his next tournament will now be the Western and Southern Open, starting on August 13 in Cincinnati.
Britain's Murray, in only his third event after an 11-month layoff with a right hip injury, edged Romania's Marius Copil 6-7 (5/7), 6-3, 7-6 (7/4) in a Thursday match that began at midnight thanks to rain delays and ended at 3:02 a.m. Friday morning - the latest finish in the tournament's 50-year history by 35 minutes.
"I'm exhausted after playing so much over the past four days, having not competed on the hard courts for 18 months", Murray said in a statement on the ATP website.
If his body can take it, Murray plays for a semifinal spot in Friday's last match against 72ndranked De Minaur, who pulled two upsets on Thursday after rain, dumping U.S. 11th seed Steve Johnson 6-4 7-5 and South Korean eighth seed Chung Hyeon 6-2 4-6 6-2. It's not good for anyone, I don't think, involved in the event.
The Washington event starts midweek men's matches typically not before 4pm, a nod to past complaints by some of having to repeatedly play in the heat of the day while bigger names had cooler conditions at night.
Third-ranked defending champion Alexander Zverev took bragging rights in his sibling rivalry, beating 15th-seeded older brother Mischa 6-3, 7-5 in their first ATP meeting. "There are lots of positives to take from this week, so I'll take some time to rest and recover and then to Cincinnati early to prepare and get ready".
He was to meet Australian Alex de Minaur in the Washington quarter-finals.
"Not just my hip, but the rest of my body as well". "(My body) doesn't feel great right now. "I'm not sure how well I'll recover from that".
On Saturday, he'll face 19-year-old Stefanos Tsitsipas of Greece.
All tennis fans rejoiced at his pain-free movement and fitness levels shown to win three such draining matches.
He will add a third in Washington if he can continue his form at the tournament where his overall record stands at 15-2. Zverev joked after advancing, "I think I'm the only one that's allowed to buy a drink in the USA, right?"
"I'm disappointed with that, because I know that the weather's tricky and I know it is for the scheduling, but it's a very hard position to be in".