The Australian, ranked 72nd in the world, defeated World No. 17 Mertens in a marathon three-hour contest that was decided by three tie-breaks – winning 7-6 (5), 6-7 (5), 7-6 (4).
With very little separating either player – who bagged four breaks apiece – the contest was decided by the finest of margins in a match that could have truly gone either way.
But in the end – after some outstanding shot making from both players – it was Tomljanović who claimed victory, much to her surprise.
“I didn’t expect to play that well,” she said. “But I came out and thought I’d just do my best. I was actually just happy that it was a great match but with the outcome I’m even happier.
“This is a really big confidence boosting win for me because I know Elise never gives you the match. She really made me work for it the whole match.
“My energy was very neutral. I wasn’t getting too happy or too sad and it worked, so maybe I should be like that all the time.
“I just love this place; I love the people. The crowd is always great. Usually I always go to the movies and to the Mall when I’m here but obviously this year that’s not the case. It’s really that and the people that keep me coming back here.”
Elsewhere No.2 seed Ons Jabeur, from Tunisia, made a winning start to the grass court season by beating American Caty McNally 6-4, 6-2 in just over an hour. Jabeur – who reached the fourth round of the French Open earlier this month – put in a strong performance, hitting four aces and winning 87 percent of her first serve points, while not facing a single break point from her opponent.
“I like playing on grass,” Jabeur said. “I like all the slicing, but it takes some time to get used to and I kind of feel like I’m on a football pitch!
“I thought I would give playing here a try. I feel like grass should be played on in England so I prefer to come here and play here.
“After Roland Garros I took two days off – I didn’t have much time – just to forget a little bit about clay and then I came here to try and get the feel of the courts. There are no grass courts in Tunisia; only football fields. Maybe I can play on those!”
Heather Watson and Harriet Dart were beaten by fellow Briton Naiktha Bains and Czech Tereza Martincova 4-6 6-3 10-5 in the Birmingham Classic doubles first round. Watson, 29, is still in the singles and faces fifth seed Zhang Shuai for a spot in the quarter-finals today.
l Meanwhile an emotional Andy Murray admitted he has been telling himself that every match could be his last after making a winning return to Queen's Club. The former world number one, back on the comeback trail following yet another injury setback, faced Benoit Paire at the cinch Championships in his first singles match on grass since 2018.
After completing a routine 6-3 6-2 victory – his first singles win on home soil in 1084 days – Murray became tearful in his courtside interview. Earlier British number one Dan Evans ended his losing streak at Queen’s with a 6-4 6-4 win over Australian youngster Alexei Popyrin. Evans, at a career-high ranking of 25 in the world and seeded sixth, had not won a match at the Wimbledon tune-up in west London since 2014.
The 31-year-old had relaxed on Monday by watching Scotland’s Euro 2020 defeat by the Czech Republic with Murray, as well as the other players.
“It’s actually saving us in the bubble a bit, the Euros.”