Kyle Edmund found it "tough" and "weird" to defeat friend and mentor Andy Murray at the Nature Valley International on Wednesday.
Murray returned from a lengthy injury lay-off at Queen's Club last week and then joined the field at Eastbourne, but his progress was cut short by Edmund, who became British number one in the three-time grand slam champion's absence.
A 6-4 6-4 victory was a strange experience for Edmund, though, having practiced with Murray as recently as the start of the week.
"It was a tough match mentally, playing Andy. I just had to try to view it as another tennis match instead of [thinking about] who I'm playing," he said.
"It's always nice to come through like that, against Andy, who I've looked up to so much. He's looked after me and really been good to me. It's a bit of a weird feeling. I got a bit nervous at the end."
Another all-British battle saw wildcard Cameron Norrie beat lucky loser Jay Clarke, but Edmund was one of only three seeds to advance on a day of upsets.
Lukas Lacko dumped out top seed Diego Schwartzman, whose fellow Argentine Leonardo Mayer lost to John Millman.
David Ferrer is out, too, going down to Mikhail Kukushkin - Edmund's next opponent - while Mischa Zverev saw off Steve Johnson.
Denis Shapovalov and French Open semi-finalist Marco Cecchinato also went through.
At the Antalya Open, defending champion Yuichi Sugita went down 2-6 6-4 7-6 (7-5) to Pierre-Hugues Herbert.
Top seeds Adrian Mannarino and Damir Dzumhur progressed, along with Gael Monfils and Joao Sousa, but Robin Haase was beaten by Guillermo Garcia-Lopez.