diff --git a/CHANGELOG.md b/CHANGELOG.md
index f1d3251a..be56e8e4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,7 +6,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## Unreleased
 
-- Add support for HTTPS proxies. Currently only available for async. (#745)
+- Add support for HTTPS proxies. (#745, # 786)
 - Handle `sni_hostname` extension with SOCKS proxy. (#774)
 - Change the type of `Extensions` from `Mapping[Str, Any]` to `MutableMapping[Str, Any]`. (#762)
 - Handle HTTP/1.1 half-closed connections gracefully. (#641)
diff --git a/httpcore/_backends/sync.py b/httpcore/_backends/sync.py
index 730795f6..f2dbd32a 100644
--- a/httpcore/_backends/sync.py
+++ b/httpcore/_backends/sync.py
@@ -2,6 +2,7 @@
 import ssl
 import sys
 import typing
+from functools import partial
 
 from .._exceptions import (
     ConnectError,
@@ -17,6 +18,103 @@
 from .base import SOCKET_OPTION, NetworkBackend, NetworkStream
 
 
+class TLSinTLSStream(NetworkStream):  # pragma: no cover
+    """
+    Because the standard `SSLContext.wrap_socket` method does
+    not work for `SSLSocket` objects, we need this class
+    to implement TLS stream using an underlying `SSLObject`
+    instance in order to support TLS on top of TLS.
+    """
+
+    # Defined in RFC 8449
+    TLS_RECORD_SIZE = 16384
+
+    def __init__(
+        self,
+        sock: socket.socket,
+        ssl_context: ssl.SSLContext,
+        server_hostname: typing.Optional[str] = None,
+        timeout: typing.Optional[float] = None,
+    ):
+        self._sock = sock
+        self._incoming = ssl.MemoryBIO()
+        self._outgoing = ssl.MemoryBIO()
+
+        self.ssl_obj = ssl_context.wrap_bio(
+            incoming=self._incoming,
+            outgoing=self._outgoing,
+            server_hostname=server_hostname,
+        )
+
+        self._sock.settimeout(timeout)
+        self._perform_io(self.ssl_obj.do_handshake)
+
+    def _perform_io(
+        self,
+        func: typing.Callable[..., typing.Any],
+    ) -> typing.Any:
+        ret = None
+
+        while True:
+            errno = None
+            try:
+                ret = func()
+            except (ssl.SSLWantReadError, ssl.SSLWantWriteError) as e:
+                errno = e.errno
+
+            self._sock.sendall(self._outgoing.read())
+
+            if errno == ssl.SSL_ERROR_WANT_READ:
+                buf = self._sock.recv(self.TLS_RECORD_SIZE)
+
+                if buf:
+                    self._incoming.write(buf)
+                else:
+                    self._incoming.write_eof()
+            if errno is None:
+                return ret
+
+    def read(self, max_bytes: int, timeout: typing.Optional[float] = None) -> bytes:
+        exc_map: ExceptionMapping = {socket.timeout: ReadTimeout, OSError: ReadError}
+        with map_exceptions(exc_map):
+            self._sock.settimeout(timeout)
+            return typing.cast(
+                bytes, self._perform_io(partial(self.ssl_obj.read, max_bytes))
+            )
+
+    def write(self, buffer: bytes, timeout: typing.Optional[float] = None) -> None:
+        exc_map: ExceptionMapping = {socket.timeout: WriteTimeout, OSError: WriteError}
+        with map_exceptions(exc_map):
+            self._sock.settimeout(timeout)
+            while buffer:
+                nsent = self._perform_io(partial(self.ssl_obj.write, buffer))
+                buffer = buffer[nsent:]
+
+    def close(self) -> None:
+        self._sock.close()
+
+    def start_tls(
+        self,
+        ssl_context: ssl.SSLContext,
+        server_hostname: typing.Optional[str] = None,
+        timeout: typing.Optional[float] = None,
+    ) -> "NetworkStream":
+        raise NotImplementedError()
+
+    def get_extra_info(self, info: str) -> typing.Any:
+        if info == "ssl_object":
+            return self.ssl_obj
+        if info == "client_addr":
+            return self._sock.getsockname()
+        if info == "server_addr":
+            return self._sock.getpeername()
+        if info == "socket":
+            return self._sock
+        if info == "is_readable":
+            return is_socket_readable(self._sock)
+        return None
+
+
 class SyncStream(NetworkStream):
     def __init__(self, sock: socket.socket) -> None:
         self._sock = sock
@@ -59,10 +157,18 @@ def start_tls(
         }
         with map_exceptions(exc_map):
             try:
-                self._sock.settimeout(timeout)
-                sock = ssl_context.wrap_socket(
-                    self._sock, server_hostname=server_hostname
-                )
+                if isinstance(self._sock, ssl.SSLSocket):  # pragma: no cover
+                    # If the underlying socket has already been upgraded
+                    # to the TLS layer (i.e. is an instance of SSLSocket),
+                    # we need some additional smarts to support TLS-in-TLS.
+                    return TLSinTLSStream(
+                        self._sock, ssl_context, server_hostname, timeout
+                    )
+                else:
+                    self._sock.settimeout(timeout)
+                    sock = ssl_context.wrap_socket(
+                        self._sock, server_hostname=server_hostname
+                    )
             except Exception as exc:  # pragma: nocover
                 self.close()
                 raise exc