|2020 Australian Open|
|Venue: Melbourne Park Dates: 20 January to 2 February|
|Coverage: Listen on BBC Radio 5 Live Sports Extra and online; Live text on selected matches on the BBC Sport website and app.|
British number one Johanna Konta made her earliest exit from the Australian Open by losing to Ons Jabeur in the first round but Harriet Dart advanced with a thrilling three-set win.
Konta, seeded 12th, was beaten 6-4 6-2 by the tricky Tunisian world number 78.
Compatriots Kyle Edmund, Cameron Norrie and Katie Boulter also lost.
Qualifier Dart, ranked 173rd, showed superb fight to win a final-set tie-break against Misaki Doi, triumphing 2-6 6-4 7-6 (10-6) at Melbourne Park.
The 23-year-old will face Wimbledon champion and 2018 Australian Open finalist Simona Halep in the second round.
Edmund fell 7-6 (9-7) 6-3 7-6 (7-4) to Serbia's 24th seed Dusan Lajovic, while Boulter pushed Ukrainian fifth seed Elina Svitolina before losing 6-4 7-5.
Norrie lost 7-5 3-6 3-6 7-5 6-4 against France's Pierre-Hugues Herbert in a near four-hour match which the British men's number two rued being "in complete control of" before losing his grip.
Konta was playing only her second match in almost five months because of a knee injury.
The 28-year-old struggled to settle as Jabeur knocked out Britain's highest-ranked player.
"It's important to recognise the season is long," Konta told BBC Sport.
"I'm not going to rush or sprint towards suddenly packing it all in. Things will come with time and putting the right work in."
Heather Watson was set to play against Czech world number 64 Kristyna Pliskova, but her match has been rescheduled for 00:00 GMT on Wednesday because of the slow progress of matches on day two.
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Rusty Konta unpicked by Jabeur
Konta, a 2016 semi-finalist at Melbourne Park, went into this year's opening Grand Slam having played only one match since September's US Open.
A tendonitis-like inflammation of the knee, which she suffered in the latter stages of last season, has had to be carefully managed.
Although she did not look troubled by the problem against Jabeur, Konta did show signs of rust and was unable to raise her game to the levels that took her to the French Open semi-finals, along with the Wimbledon and US Open quarter-finals, last year.
Konta, who said afterwards she was "pleased" with how her knee felt, hit 19 unforced errors, while Jabeur punished her with 19 winners.
The Briton's serve came under immediate pressure from Jabeur, who is able to unsettle opponents with her variety, and she had to save a break point in the opening game.
After steadying herself, an erratic game where she struggled on her first serve enabled Jabeur to strike and take the opening set.
Konta, backed by a healthy number of British fans, came out for the second set with renewed purpose, playing more aggressively to break in the opening game.
However, she was unable to back that up with a hold and from that point Jabeur took control to win in just one hour and two minutes.
Dart battles into second round
Dart made her first main draw appearance at the Australian Open last year, where she was thrashed 6-0 6-0 by Maria Sharapova, but she showed determination to beat Japan's Doi.
The only Briton to make it through qualifying, Dart had an early break against her 82nd-ranked opponent but lost eight games in a row to go a set and a break down.
There were 15 breaks of serve as both players struggled with nerves, and Dart might have thought her chance had gone as she let two match points slip at 6-5 in the final set.
Doi quickly took a 5-1 lead in the deciding tie-break but Dart rallied, winning nine of the final 10 points to reach the second round in Melbourne for the first time after a match lasting two hours and 49 minutes.
"I'm really proud I was able to dig deep," said Dart.
"It was playing in my mind a little bit but I thought she upped her game and I tried to tell myself to stay composed and be aggressive.
"It's a long tie-break so I told myself to pace myself and keep it together emotionally. I stuck to my gameplan and I was very composed."
Enforced interruption costs Edmund
Like Konta, Edmund is a former semi-finalist at Melbourne Park and his run to the last four in 2018 continues to be an anomaly here.
This was Edmund's fourth first-round exit in six appearances, with the other being a second-round defeat in 2017.
The 25-year-old Yorkshireman was leading 5-2 in the opening set on Monday before the match was postponed following the deluge of rain which decimated day one's play.
Passive play from the world number 65 allowed Lajovic to instantly fight back to 5-5 when they resumed in bright sunshine at Tuesday lunchtime.
The Serbian player, backed by a raucous support which attempted to unsettle Edmund, edged a tight opener in a tie-break where a net cord worked heavily in his favour when the Briton was a mini-break up.
From that point Edmund struggled to cope with Lajovic's physicality in the rallies, the Serb hitting 39 winners and drawing plenty of errors from his opponent's racquet.
Once Edmund fell two sets behind, history suggested a comeback - like his compatriot Dan Evans managed on Monday - was unlikely.
Edmund had never won a match after falling two sets behind - nor even claimed a set back - and suffered another straight-set defeat when Lajovic took the third-set tie-break.
"I just couldn't get into the match and he played well. I have to learn from it," Edmund told BBC Sport.
Norrie, ranked three places ahead of Edmund at 62, may be left to rue his missed chances, having led his French opponent by two sets to one.
He recovered from an early break down in the final set but, serving to take the decider to 5-5, faltered, allowing Hubert to break again and ultimately serve out victory in three hours and 42 minutes.
Boulter runs fifth seed Svitolina close
Despite playing her first top-level match since April last year following a back injury, Boulter pushed a player ranked among the favourites to win the women's singles title.
The 23-year-old suffered the injury in Great Britain's Fed Cup tie against Kazakhstan, playing through the pain barrier to win her two singles matches.
That aggravated the problem, however, and curtailed the then 86th-ranked player's season until a comeback on the second-tier ITF tour in November.
Boulter has slipped to 317th as a result, but little separated the pair in a tight match.
Svitolina had only won three points more than her opponent until the Briton lost serve to love when trying to stay in the opening set.
Boulter broke immediately in the second set, rocking Svitolina with a pair of forehand winners after the Ukrainian opened the door with a double fault.
Boulter managed to back up the break with a hold, only to lose the advantage in her next game as the lack of top-level matches suddenly seemed to catch up with her.
However, she again fought back to level at 5-5 before Svitolina's quality and nous saw her through to the second round.
The Ukrainian again broke when Boulter served to stay alive, a blistering forehand winner down the line clinching her second match point.
"I think I played some really good points but I also gave a lot away, which for me is quite disappointing because I expect better of myself. But it was a good match, I have to take the positives from it," said Boulter.
"Any match is 50-50, I feel like I can win a lot of matches at this level and I hope I can prove that."
BBC tennis correspondent Russell Fuller
Johanna Konta looked a long way off the pace in what was only her second match since 4 September.
She could have played in Adelaide last week, but took the decision to build up the load on her troublesome knee more gradually.
In fact, listening to her post match, it is clear she was not 100% sure she would definitely play in Melbourne when she boarded the plane to Australia.
Konta will now have to be patient. A first-round exit here, coupled with her decision to skip the Fed Cup in 2020, means the earliest she could return to action would be in St Petersburg in three weeks' time.
However, it may be too late to get a spot in the draw, even if she wants one, with Konta's next scheduled tournament not starting in Dubai until 17 February.
As for Ons Jabeur, she played with admirable control - like a higher-ranked player dealing efficiently with a tricky opponent.