First performed by Herb Alpert (1968).
First released (as “That Guy’s in Love”) by Danny Williams (1968).
Hit versions by Herb Alpert (US #1/MOR #1/CAN #1/UK #3/AUS #1 1968), Dionne Warwick (as “This Girl’s in Love with You” US #7/MOR #2/R&B #7/CAN #7 1969).
From the wiki:”‘This Guy’s in Love with You’ was by Burt Bacharach and Hal David. [T]he original performance originated when Herb Alpert, while visiting at Bacharach’s office, asked, ‘Say, Burt, do you happen to have any old compositions lying around that you and Hal never recorded; maybe one I might be able to use?’ Alpert said he made it his practice to ask songwriters that particular question: often a ‘lost pearl’ was revealed. As it happened, Bacharach recalled one, found the lyrics and score sheet in his office filing cabinet, and offered it to Alpert: ‘Here, Herb … you might like this one.’
“Alpert originally sang ‘This Guy’s in Love with You’ on his April 1968 television special, The Beat of the Brass. In response to numerous viewer telephone calls to the network following the broadcast, Alpert decided that the song should be used as the promotional single for the subsequent May 1968 release of the TV special’s soundtrack. But, the first release of ‘This Guy’s in Love with You’, titled ‘That Guy’s in Love’, was in the UK by South African-born singer Danny Williams in late April 1968, for his self-titled album. Williams’ recording, however, was not released as a single.
“Alpert’s single recording reached #1 on the U.S. Billboard Hot 100 pop singles chart in June 1968, remaining in the top position for four weeks. (The album, too, was a pop album #1 hit.) It was not only Alpert’s first#1 single, but it was also the first #1 single for Alpert’s A&M record label. Eleven years later Alpert became the first (and only) artist to have reached the prized No. 1 position of the Billboard Hot 100 with both a vocal performance and an instrumental performance when his instrumental, ‘Rise’, reached the top of the hit chart.
“Dionne Warwick’s version, titled ‘This Girl’s in Love with You’, was released as a single in 1969, reaching #7 in the U.S. that same year. It also spent four weeks at #2 on the Easy Listening chart. It ranked as the 64th biggest US hit of 1969. As a demo, Warwick’s version was actually the original as it preceded Alpert’s. ”
Herb Alpert, “This Guy’s in Love with You” from Beat of the Brass TV special (1968):
Herb Alpert, “This Guy’s in Love with You” (1968):
Dionne Warwick, “This Girl’s in Love with You” (1969):