Never count out Elise Mertens.

After over two hours of gruelling tennis, the No.10 seed stared down a 5-7, 2-5 deficit against Jessica Pegula in the Dubai Duty Free Tennis Championships quarterfinals. Eleven games later, Mertens walked off court the winner, having saved three match points serving at 4-5 in the second set.

"I just didn't want to let it go," Mertens declared afterwards. "I kept on fighting and that was the spirit today."

The Belgian not only demonstrated indomitable will but also one of the finest tactical brains on the Tour and some hefty endurance levels. The two-hour, 47-minute contest was the longest of the tournament so far, and comprised numerous absorbing momentum shifts as Mertens and Pegula forced each other to keep adjusting.

Varying pace and spin, Mertens had the better of an edgy opening, grinding into a 4-2 lead as Pegula was kept off-balance. The American's solution was to come forward: committing to a serve-and-volley approach saw Pegula survive a nine-minute hold and ultimately snatch the first set. Impressively, she stuck to the strategy even when Mertens had seemingly gotten wise to it with a pair of brilliant backhand passes.

The first set alone had taken 75 minutes, and unsurprisingly Pegula surged after capturing it. Mertens, beset by an uncharacteristic amount of errors and 13 double faults, barely clung on as Pegula advanced to the brink of victory.

But with her back to the wall, the Gippsland Trophy champion became watertight. Sensing tension creep into the Pegula forehand, Mertens directed all traffic to that wing on each match point she faced, and was rewarded with an error over the baseline three times.

Having levelled at 5-5, a rejuvenated Mertens worked a fading Pegula relentlessly around the court, exploiting the World No.36's increasing signs of fatigue as she raced through the last 11 straight games.

Earlier, Barbora Krejcikova's singles surge showed no signs of slowing in the day's first match, a 6-0, 6-2 demolition of Anastasia Potapova in 62 minutes.

The former doubles World No.1 is the owner of five Grand Slam titles - two in women's doubles and an Australian Open hat-trick in mixed. However, Krejcikova's focus on a high-level doubles schedule came at the expense of playing tournaments that would have enabled a singles breakthrough.

Only last October did the Czech crack the Top 100 in singles after a run to the Roland Garros fourth round - but ever since, she's been proving she belongs in this territory.

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A second career semifinal followed in Linz at the end of 2020, followed by a Grampians Trophy quarterfinal showing last month. This result marks Krejcikova's debut in the last four of a WTA 1000 event.

Throughout, World No.63 Krejcikova was cool-headed and in control. The 25-year-old's backhand was particularly impressive: she was able to find winning angles as well as redirect it down the line with ease.

By contrast, Potapova began shakily and never settled. The teenager had been forced to retire from her doubles second round the previous night after being hit in the eye with a ball, and two double faults in her first service game unfortunately set the tone. There would be seven from the Russian in total, including another three to drop serve for the first time in the second set.

There were also 28 unforced errors, frequently on standard rally balls, prompting the 19-year-old to berate herself in anguish: "The court is there!"

"I'm just so relieved and so happy that I was able to play my best tennis," said Krejcikova after closing out the win to love with four unreturned serves. She will bid for her second career WTA final following Nurnberg 2017 against another unseeded player, Jil Teichmann.