Andy Murray was knocked out of the San Diego Open, while Cameron Norrie came out on top in the all-British clash against Dan Evans.
The former world No 1, given a wild card entry into the tournament as he continues his latest comeback from hip surgery and other injuries, lost to second seed Casper Ruud 7-5 6-4 in an hour and 49 minutes.
Currently ranked 109th, Murray cruised through the first round with a straight-sets win over lucky loser Denis Kudla of the US.
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The Norwegian, who won four tournaments on clay in Europe earlier this year, later praised Murray for making his comeback, saying he had played "at a high level" in their match.
"On the opposite side of the net from me was a legend of the game for the past decade or so, even what he's done now for the past year, fighting his way back from very serious injury has been an inspiration for me," Ruud said in his on-court interview, broadcast on ATP Tennis TV.
"For the previous 10 or 15 years, he's been an inspiration to watch on TV, the way he fights and always finds a way to never give up.
"The way he plays has been amazing. It's been an unfortunate couple of years for him but it's great to see him back and I think he was playing at a high level today."
Meanwhile, Murray, who reached the quarter-finals of last week's Moselle Open in Metz, has been handed a wild card into next week's BNP Paribas Open in Indian Wells.
Norrie came from behind in the opening set to sink compatriot Evans in their San Diego showdown.
Norrie won the battle of Britain's top two-ranked players 7-6 (7-3) 6-3 after hitting back from 5-2 down and saving two set points in the opener.
It maintained Norrie's fine good record against eighth seed Evans, having also won their only previous tour clash in the first round of this year's Australian Open.
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Canadian Denis Shapovalov is through to the quarter-finals against Norrie after powering away from American Taylor Fritz 7-6 (7) 6-2.
Russian world No 24 Aslan Karatsev caused a minor upset by beating fifth seed Hubert Hurkacz of Poland 5-7 6-4 6-2 and will now play 29th-ranked Bulgarian Grigor Dimitrov, who cruised past lucky loser August Holmgren of Denmark 6-1 6-1, and Italian ninth seed Lorenzo Sonego defeated American Sebastian Korda 6-4 6-3.
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