John McEnroe remains cautious about Andy Murray's hopes of a successful singles comeback.
The former world number one made a brilliant return to the match court at Queen's Club last week following a second hip operation in January, winning the doubles title with Feliciano Lopez.
Murray has also restricted himself to doubles at Wimbledon but spoke about the possibility of a singles return as early as the US Open at the end of August.
McEnroe is thrilled to see Murray back but is not reading too much into his doubles achievements.
Speaking on behalf of espn.co.uk, the American said: "Doubles is obviously a completely different animal than singles. Best of five (sets) is totally different than best of three.
"But it was nice to see him eager and into it and looking like he was relatively healthy. You can't really determine it until he starts playing singles matches and how the body reacts if you're on a hard court, for example, as opposed to grass.
"It looked very promising, which is great, and I think everyone wants him to be able to come back and then be able to retire on his own terms."
In Murray's absence from singles, British hopes of a title challenger rest with Johanna Konta following her surprise run to the French Open semi-finals.
American great Chris Evert declared herself speechless at Konta's performance in her quarter-final victory over Sloane Stephens, but the British number one was ultimately disappointing in a semi-final loss to Marketa Vondrousova where her old problems with nerves resurfaced.
"I was calling a match at the French, and she could have won the tournament the way she played, and then I see her play a match where she could have lost to almost anybody," said Evert.
"I think she's still a work in progress with the mental side of the game, but I think she's working really hard at it. Just the way she's wired, she has to work a little bit more on taking the emotion out of the game and just being fully focused."
In the women's tournament, a lot of focus will again be on the form of Serena Williams as she continues her pursuit of Margaret Court's all-time record of 24 grand slam singles titles.
The American reached the finals of Wimbledon and the US Open last year but this season has been disappointing, with the 37-year-old hampered by a knee problem and desperately short of matches.
Williams did not follow through with her suggestion that she might play a warm-up tournament and arrives at the All England Club having completed just six matches since January.
Evert feels Williams made a mistake not playing in Birmingham or Eastbourne, saying: "I think definitely she should have done, just so she can gauge where she is on the grass and what she needs to work on.
"Maybe she doesn't need to be 100 per cent, but she certainly needs to be 80, 90 per cent to really have a chance.
"The one difference that I see in Serena is since she's come back, she's been one or two steps slower than normal. I think that's a little bit of fitness. I think it's a little bit of match play. I think it's a little bit of confidence.
"But it's hard for me to bet against Serena. I would have said in January, if there was any grand slam she was going to win, it would probably be Wimbledon, and I just think with her game, she just loves hitting the ball. She just loves that power. And, if her serve is on, she's going to be tough to beat."
McEnroe feels the pressure of time will be playing on the mind of Williams, who turns 38 in September.
"As you get older, at least for me, I felt there was more pressure because you realise that your window is closing and you don't know how much longer you're going to be doing it," he said.
"I don't know what she's thinking. But I'm thinking she can't be thinking, 'I'm going to be playing much more than a year or two'."
