Andy Murray battled hard as he continued his comeback to tennis, but was defeated in four sets by Fernando Verdasco in Round 2 at the 2018 U.S. Open on Wednesday. 

The former men's champion fell to the 34-year-old Spaniard in sweltering conditions, 7-5, 2-6, 6-4, 6-4.

Defending women's champion Sloane Stephens booked a place in the third round after overpowering Anhelina Kalinina after an early scare.

The No. 3 seed took the victory after dropping the opening set, 4-6, 7-5, 6-2.

Serena Williams will join her in the next round after defeating Carina Witthoeft in straight sets and dropping just four total games.

On the men's side, Juan Martin del Potro continued his fine form this season, defeating Denis Kudla 6-3, 6-1, 7-6 (4), while Rafael Nadal advanced with a straight sets win over Vasek Pospisil.

Here are a selection of results from Wednesday's action in New York:

Men's Singles - Round 2

(3) Juan Martin del Potro def. Denis Kudla 6-3, 6-1, 7-6 (4)

Stan Wawrinka def. Ugo Humbert 7-6 (5), 4-6, 6-3, 7-5

(25) Milos Raonic def. Gilles Simon 6-3, 6-4, 6-4

(31) Fernando Verdasco def. Andy Murray 7-5, 2-6, 6-4, 6-4

(9) Dominic Thiem def. Steve Johnson 6-7 (5), 6-3, 5-7, 6-4, 6-1

(5) Kevin Anderson def. Jeremy Chardy 6-2, 6-4, 6-4

(11) John Isner def. Nicolas Jarry 6-7 (7), 6-4, 3-6, 7-6 (2), 6-4

(1) Rafael Nadal def. Vasek Pospisil 6-3, 6-4, 6-2

Women's Singles - Round 2

(3) Sloane Stephens def. Anhelina Kalinina 4-6, 7-5, 6-2

(16)  Venus Williams def. Fernando Verdasco 6-4, 7-5

(7) Elina Svitolina def. Tatiana Maria 6-2, 6-3

(8) Karolina Pliskova def. Ana Bogdan 6-2, 6-3

Ekaterina Makarova  def. (9) Julia Goerges 7-6 (10), 6-3

(17) Serena Williams def. Carina Witthoeft 6-2, 6-2

Full results and schedule available via the U.S. Open's official website. 

Recap

Murray gave it a huge effort as he fights his way back to full fitness after injury, but the British superstar didn't have the legs to beat Verdasco.

The former Wimbledon champion will be encouraged by his display, but the length of the match played into the hands of his opponent.

Verdasco edged the opening set 7-5, but Murray responded by claiming the second 6-2.

Murray's serve appeared solid throughout the encounter, producing nine aces as he tried to regain his rhythm against the No. 31 seed.

However, the Scot wained in the heat of the day, and his opponent's superior fitness was telling.

Verdasco won the third and fourth sets, 6-4, and grabbed a place in the third round.

Former British No. 1 Andrew Castle was impressed by Murray's performance as he continues his recovery:

Stephens proved she will continue to be a force in the tournament, rushing to a three-set win over Kalinina.

The reigning women's champion took her time to find her groove but proved too good once she found the accelerator.

The American hit 31 winners, as her Ukrainian opponent fought to keep the encounter alive.

Kalinina had won the opener 6-4, but Stephens was assured as she won the second set 7-5 and final set 6-2.

There were also victories for women's seeds Elina Svitolina, Karolina Pliskova and Venus Williams.

The trio made little fuss as they claimed straight-set wins to proceed into the next round, and Williams' victory helped set up a third-round showdown with her sister, Serena, after the 23-time Grand Slam winner handled Witthoeft with relative ease.

Juan Martin del Potro was also a straight-sets winner, with the men's third seed beating American talent Kudla. Nadal was as well, keeping alive the possibility of a Nadal-Roger Federer championship match.

It was an emotional day for Murray, but he proved he could recapture his very best form in the months ahead.

The Brit extended himself during each and every point, and free of his physical restrictions, he will once again contest for the biggest prizes on the tour.

All stats via U.S. Open's official website.