US Open champion Carlos Alcaraz comes unstuck against David Goffin at Astana Open

Alcaraz was playing his first tour event since lifting his maiden grand slam title at the US Open last month and becoming the youngest ever men's world number one.

But the 19-year-old struggled on serve throughout the contest with former top-10 player Goffin, who earned a lucky loser place after falling in qualifying, and was beaten 7-5 6-3.

Remarkably, it was the first match this year in which Alcaraz has failed to win at least a set.

The Spaniard looked a little subdued throughout and made 34 unforced errors, dropping serve five times.

Alcaraz said: "It was a tough match. David was playing really, really well. To come back to competition is never easy. He played two matches here on this court, which is not easy to get used to. It's really, really slow. It's really tough for me, for my confidence.

"I tried to get used to the court so quick but I couldn't. He was better than me. I couldn't handle the pressure he was putting on me. It's something that I have to learn and take the lesson from this match."

Goffin, currently ranked down at 66, has had a tough run of results since a five-set loss to Cameron Norrie in the Wimbledon quarter-finals, and he said: "It means a lot, just to prove to myself that my tennis is still there, I can win some big matches like today."

Third seed Stefanos Tsitsipas fared better, beating Kazakhstan's Mikhail Kukushkin 6-3 6-4, while Karen Khachanov saw off Maxime Cressy 6-4 7-6 (3).

At the Japan Open in Tokyo, top seed Casper Ruud, who was beaten by Alcaraz in the US Open final, admitted his heavy schedule had caught up with him after a 6-3 6-3 loss to Spain's Jaume Munar.

Britain's Dan Evans came from a set down to defeat Radu Albot 6-7 (3) 6-1 6-4 while Nick Kyrgios was in impressive form on his return to action, beating Tseng Chun-hsin 6-3 6-1.

Kyrgios had hinted after his painful loss to Khachanov in the US Open quarter-finals that he might shut down his season but he has regrouped and will look to push into the top 20 this week.

The 27-year-old said: "I'm expecting some big things from myself for the rest of the year. I don't want to coast and wait until the (Australian) Open and just enjoy time at home, I want to stay on the road and continue to push myself and see what results I can bring.

"After the Wimbledon final and after my run at the US Open, I feel like I should be playing that type of tennis all the time. It's probably not realistic but I'm not a realistic person."

The Australian's victory came on the day his lawyer appeared on his behalf at a court in his home city of Canberra to reveal Kyrgios will apply to have an assault charge against a former girlfriend dismissed on mental health grounds.

Kyrgios is set to appear in person at the next hearing in February.

Kyrgios was the only Australian winner, with sixth seed Alex De Minaur beaten 6-3 6-2 by Kwon Soonwoo and Thanasi Kokkinakis falling 6-4 7-6 (4) to ninth seed Borna Coric.