British number one Johanna Konta made her earliest exit from the Australian Open by losing to Tunisia's Ons Jabeur in the first round.
Konta, seeded 12th, was beaten 6-4 6-2 by the tricky world number 78 at Melbourne Park.
Compatriot Kyle Edmund was also knocked out with a 7-6 (9-7) 6-3 7-6 (7-4) loss to Serbia's 24th seed Dusan Lajovic.
Katie Boulter pushed Ukrainian fifth seed Elina Svitolina before losing 6-4 7-5.
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."
Two more Britons - Harriet Dart and Cameron Norrie - play later on Tuesday, which had 96 first-round matches scheduled after the opening day was badly affected by heavy rain.
Heather Watson was also set to play on Tuesday against Czech world number 64 Kristyna Pliskova, but her match - sixth on court 10 - has been rescheduled to Wednesday because of the slow progress of the matches before.
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 take 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 also 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.
Like Konta, Edmund is also 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 the day's play.
Edmund paid the price for the enforced interruption.
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.
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 played in the late evening sunshine.
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.
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 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 Jabeur, she played with admirable control - like a higher-ranked player dealing efficiently with a tricky opponent.