It was back in June that Bublik beat Sinner on a grass court in Halle and now former Grand Slam doubles champion Ryan Harrison has hinted that the match could have been more significant than we dared to imagine.

Bublik's win was dismissed as a natural reaction for Sinner, coming a few days after his heartbreaking French Open defeat against Carlos Alcaraz in Paris.

Yet he backed that defeat with a match against Grigor Dimitrov at Wimbledon that looked set to end in defeat until the Bulgarian picked up an injury that forced him to retire from the match.

Sinner's dominant win in the Wimbledon final against Alcaraz banished any notion that players had started to uncover a way to unsettle the world No 1, but fresh question marks were raised after a lacklustre display at the US Open against Denis Shapovalov.

The defending champion has not lost a hard court Grand Slam match for two years, but he was in real peril at 3-0 down in the third set with the pair locked at one-set all.

Shapovalov had won their only previous meeting at the Australian Open four years ago and he had a chance to move 4-0 ahead in that second set before Sinner stormed back to win 5-7, 6-4, 6-3, 6-3.

Yet as the Italian reflected on his passage to the last-16 in New York, he admitted he had been pushed all the way.

"It was a very, very tough match today," said Sinner. "I know Denis for quite a long time. I knew I had to play at a very, very high level. I'm very happy that I managed to win. He started off very well. I just tried to stay there mentally."

"He started off very well, so I just tried to stay there mentally trying to see what's coming. The last time we played was some years ago and I feel like we've both improved so much.

"I knew that I had to play at a very, very high level today so I'm very happy that I managed to win."

In his press conference, he insisted he was not surprised by his struggled against Shapovalova as he added: "I'm not a machine.

"I also struggle sometimes. Today I felt like from my side I didn't struggle. I was playing great tennis, and he also."

Sinner was a relieved man as he wriggled out of a mini-crisis against Shapovalov, but former French Open doubles champion Harrison believes the locked room may finally start to see some cracks in the game of the four-time Grand Slam champion.

After throwing in 38 unforced errors against Shapovalov, Sinner's inconsistency will inspire future opponents to believe he can be beaten, but Harrison is among those convinced the mood music around the defending champion may be changing.

"I thought Shapo's game plan was really good going out there. We're getting a little bit of a book on guys giving Sinner some trouble," Harrison told Sky Sports.

"Bublik a little while ago had the variety on the backhand side and then at Wimbledon when he struggled with Dimitrov, a guy who uses a lot of variety. Guys are not wanting to stay in rallies with Sinner, because when they do they don't win."

Tennis legend Martina Navratilova also gave her verdict on Sinner, as she claimed he will be worried by the display against Sahpovalov.

"Shapovalov played tennis like he used to and now he's healthy and Jannik wasn't hitting the ball as well or looking like himself until the end of the match," said the 18-time Grand Slam singles champion.

"He got himself out of trouble, but he must be worried a little bit."

Sinner bounced back from his lucky escape against Dimitrov to win Wimbledon in emphatic fashion and he may well do the same at the US Open, but any suggestion that the reigning champ is unbeatable can now be banished after this uncertain display against Shapovalov.

READ MORE: Jelena Ostapenko apologises to Taylor Townsend following racism row at US Open

SPAR CHALLENGE

Proteas coach Jenny Van Dyk set for her touchline debut against Malawi

Editors Pick