Andy Murray's stay in San Diego was short-lived, as the veteran fell to second seed Casper Ruud in straight sets Thursday.
World number 10 Ruud prevailed 7-5 6-4 over the three-time grand slam winner, rolling to victory after a shaky start.
Murray broke Ruud in the 22-year-old's second service game of the match, but could not maintain the advantage as Ruud converted five of six break points in the match.
Ruud will face ninth seed Lorenzo Sonego, who defeated Sebastian Korda 6-4 6-3, saving six of seven break points against his own serve while converting all three of his chances on Korda's.
Fourth seed Denis Shapovalov defeated Taylor Fritz 7-6 (9-7) 6-2, saving six set points in the process.
The Canadian moves on to a quarter-final meeting with Cameron Norrie, who downed Daniel Evans 7-6 (7-3) 6-3.
Grigor Dimitrov dominated August Holmgren 6-1 6-1 in 56 minutes, winning 83 per cent of his service points.
Dimitrov next faces Aslan Karatsev, who rallied to upset fifth seed Hubert Hurkacz 5-7 6-4 6-2.