Raducanu has not won three matches in a row since her US Open triumph last yearEmma Raducanu's dominant form at the Cincinnati Open ended as seventh seed Jessica Pegula proved a sharp step up in class for the British number one.
Raducanu had trounced veterans Serena Williams and Victoria Azarenka, but lost 7-5 6-4 to Pegula in the last 16.
Now the 19-year-old switches her focus to defending her crown at the US Open, which starts in New York on 29 August.
Cameron Norrie continued the British interest by beating Ben Shelton to reach the men's quarter-finals.
British men's number one Norrie, seeded ninth, won 6-0 6-2 against the teenage American wildcard as the gulf between the pair showed.
Norrie, who beat Andy Murray in the previous round, secured victory in just 57 minutes and will play Spain's Carlos Alcaraz, who is seeded third, in the quarter-finals.
Defeat but renewed hope for Raducanu after positive week
Optimism around Raducanu increased after her statement wins against 23-time Grand Slam champion Williams and fellow major winner Azarenka, albeit with the caveat that both of her ageing opponents are nowhere near their prime.
Against Williams, the world number 13 produced probably her best performance since her shock win at the US Open last year, backing that up with another confident display against an out-of-sorts Azarenka.
Dropping just six games combined in the two victories - each lasting a little over an hour - was a welcome boost for Raducanu after a tough year.
But facing the consistent Pegula was always likely to provide a better assessment of where the Briton's game is.
It was only the second time in Raducanu's burgeoning career that she has faced a top-10 ranked opponent and, in Pegula, one who has racked up the second highest number of WTA 1000 match wins since the start of 2021.
After losing serve in the third game of the match, Raducanu broke straight back but continued to be put under pressure by Pegula's heavier ball-striking.
Raducanu fought off three more break points to hold for 3-2, and another in the ninth game, before Pegula finally took another chance for a 6-5 lead.
That left the 28-year-old American serving for the opening set and she took the chance without conceding a point.
Recent defeats in Washington and Toronto saw Raducanu edged out in tight opening sets before fading quickly against both Liudmila Samsonova and Camila Giorgi.
Raducanu hit an untimely double fault to fall a break down early in the second set against Pegula and while her opponent's level remained slightly higher, showed resilience to stay within a single break.
The narrow lead was enough for the impressive Pegula - who is enjoying her breakout year after struggling with injuries at the start of her career - to serve out the match without facing too much pressure.
As many predicted, Raducanu's first full season on the WTA Tour has been a learning curve, having to deal with a string of injury setbacks in what is already a tough adjustment for young players to make.
However, she will return to Flushing Meadows with renewed hope after a productive and confidence-boosting week in Cincinnati.
Top seed Swiatek loses, but Medvedev through
Elsewhere in the WTA draw, top seed Iga Swiatek lost in the third round for the second successive tournament.
French Open champion Swiatek, 21, was beaten 6-3 6-1 by American Madison Keys, who earned the first win of her career against a world number one.
Poland's Swiatek has lost four of her past eight matches since her 37-match winning streak was ended at Wimbledon.
Keys will face Elena Rybakina in the quarter-finals after the Wimbledon champion beat American Alison Riske-Amritraj 6-2 6-4.
Czech Petra Kvitova won 6-1 4-6 6-0 against Tunisia's Wimbledon runner-up Ons Jabeur, while Estonian second seed Anett Kontaveit was knocked out by China's Zhang Shuai who triumphed 2-6 6-4 6-4.
In the men's draw, world number one Daniil Medvedev wore down Canada's Denis Shapovalov 7-5 7-5 to reach the quarter-finals before he starts his US Open title defence.
The Russian faces American Taylor Fritz, who beat Medvedev's compatriot Andrey Rublev 6-7 (4-7) 6-2 7-5.
Greek fourth seed Stefanos Tsitsipas, Canadian seventh seed Felix Auger-Aliassime and American John Isner are also through, along with Croatia's Borna Coric who followed up his win over Rafael Nadal by beating another Spaniard Roberto Bautista Agut.
