Britain's Dan Evans says he is "the most confident I have been on grass for a long time" after sealing his second win over a top-50 player in as many weeks.
The 35-year-old, playing as a wildcard entry at Eastbourne, beat world number 49 Miomir Kecmanovic 3-6 6-4 6-4 in two hours and nine minutes.
It provides further encouragement for the former British number one, who recently regained his place in the world's top 200 and has been given a Wimbledon wildcard.
After beating world number 13 Francis Tiafoe at Queen's last week, Evans followed that up with a hard-fought victory over Serbia's Kecmanovic in the south-coast sunshine.
"In whatever time I have left it is a work in progress," Evans said of his recent performances.
"Sometimes it takes time. I have been battling away to get levels on court, but this is the most confident I have been on grass for a long time."
Kecmanovic broke Evans twice in the 39-minute first set, but the Briton hit back at the start of the second, and celebrated with a massive fist pump.
Evans raised his game in the second set, getting 80% of his first serves in compared with 63% in the first and recording four aces. After being denied three set points, he took the fourth to force a decider.
He needed treatment to his right foot before third set commenced, but then broke Kecmanovic in the first game with a timely stroke of luck - the ball catching the net cord and dropping dead over the other side as the Serb could only look on exasperated.
Both men were below their best in the decider exchanging four successive breaks of serve, however Evans eventually held his nerve to serve out the match.
Ultimately he was more consistent than his higher-ranked opponent - Kecmanovic hit 33 winners to Evans' 22, but the Serb's 42 unforced errors were almost double the number made by the Briton.
Evans will play American second seed Tommy Paul in the next round.
It was good news for the Britons in the women's singles too, as UK number five Francesca Jones beat Belgium's Greet Minnen 6-4 7-5.
Earlier at Eastbourne, highly rated Brazilian teenager Joao Fonseca claimed his first ATP Tour win on grass by beating Zizou Bergs of Belgium in three sets.
Fonseca, 18, recovered from a bruising tie-break defeat in the opening set to claim nine consecutive games on his way to beating Bergs 6-7 6-0 6-3 in two hours and seven minutes.
The hype is continuing around the youngster labelled Brazil's newest sporting superstar. Fonseca, ranked a career-high 57th in the world, attracted global attention by beating Andrey Rublev at the Australian Open in January and has admirers including Novak Djokovic.
He will have another chance to test himself against the world's best in the second round at Eastbourne, where he will face reigning champion and top seed Taylor Fritz.
"I'm very happy to win my first match on grass," Fonseca said after his victory. "Eastbourne is such a nice place.
"In the second set, he was in trouble and I was more confident. In the third we were both nervous, so I just needed to focus on my serve.
"Playing Fritz is good experience, and the crowd is good - I see some Brazilian shirts, which is really nice."
The first set lasted 66 minutes and was dominated by the serve without a single break point until the tie-break, which Bergs won 10-8.
However the Belgian's level significantly dropped in the second set as Fonseca raced to 6-0 in 24 minutes, scoring 25 points to Bergs' eight.
Bergs briefly halted Fonseca's momentum in the deciding set, but a perfectly measured half-volley forehand dink across court moved the teenager 4-2 ahead before he served out the match.
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