A leaner Mardy Fish looks back at pictures of himself from as recently as last year and feels embarrassed.

The 28-year-old American has unloaded more than 13kg since last season, and the lighter frame has helped turn him into a heavyweight on the men's tour and rocketed him from 108th in the world to the 19th seed at Flushing Meadows.

Fish overcame a punishing first-round five-set encounter against Jan Hajek at the US Open on Tuesday, and afterwards praised a gruelling pre-season training program for enabling him to finally realise his talent.

Before this season, Fish boasted just three titles on the men's tour. But he added two more in 2010, with the impressive scalps of Andy Murray and Andy Roddick along the way.

Now, Fish said he wonders what his wife Stacey was thinking when she decided to marry him.

"I look at pictures. Some pictures I look okay, some not. It is almost embarrassing as a professional athlete," he said after his victory.

Knee surgery prompted Fish to shed the kilos and cut out a passion for burgers and pizza, a decision that has transformed him as both a player and a person.

"I feel like a completely different person and I'm playing like a completely different player," he said. "I'm able to do things that I've never been able to do before."

The new diet and training regime has not been plain sailing, and Fish is occasionally tempted to revisit his old ways.

"I saw a Domino's Pizza ad the other day - that looked really, really good," he said.

The impact of Fish's new abstemious lifestyle led five-time US Open winner Jimmy Connors to describe his impressive form as "better late than never".

"It's hard to dwell on some of the mistakes I've made, and I'm not saying they're mistakes as I've had a great life and had a lot of fun," said Fish.